Vehicle Types
Before you decide to build a faired bicycle, you need to decide what you will use it for. The design of a bike intended for use as a commuter is completely different than that of one designed for straight line speed. Because of this I will discuss three basic types of streamliner:
1) The practical streamliner or velomobile.
2) The club racer streamliner
3) The speedbike streamliner
In addition, I will discuss the two major aspects of these vehicles, the fairing, and the chassis (the bike!)

The rider of each these vehicles has very different set of criteria that his vehicle must meet.

The Practical Vehicle

The holy grail for most of us recumbent heads is a bike that you can ride around at highway speed, without breaking a sweat, zipping in and out of traffic without a care in the world. Unfortunately, that's not going to happen. What we can do is build a lightweight, reasonably weatherproof vehicle that allows safe riding in the 30 MPH range. The velomobile can be a bike or trike configuration, and each has its benefits and shortfalls.

Chassis Considerations
It is recommended to use a seat height at about the same height as a car's seat height in a velomobile, or 16-18 inches. The seat back should be more upright, above about 45 degrees, to allow turning of the body to ease the view of your surroundings. Extremely relaxed seat angles make it hard to look behind you and will cause blind spots. It is possible to use production bikes or trikes for a velomobile chassis. These vehicles should have bottom brackets higher than the seats to minimize the fairing height. Most production trikes are good candidates, but ones with a narrower track are better. production quazi-low bikes such as the Challenge Jester can be used with the seat in an upright position. The tiller steering should be minimized or eliminated if possible as it will strike the insides of the fairing in a turn. Options to tiller are "T" bar steering with no tiller (usually just for the vertically challenged), remote steering, and U-joint steering. Vehicle wheelbase should be at least 48 inches to provide decent handling. A shorter wheelbase is hard to deal with at high speeds. Trail is also important, trail should be at least 2 inches (with a straight fork) to allow the bike to handle well at high speeds. Wheel disks are recommended on the rear wheel only to prevent handling issues in high winds. Fenders or wheel spats should be used if the bike is to be ridden in the rain. If no fenders are used the rider will get just as wet as if he was out in the rain (but dirtier) because of all the wheel spray.

Trikes as chassis
The trike is nice because it is inherently stable. You can start and stop without having to put a foot down. You can ride with impunity under slippery conditions. You seal it up completely. Unfortunately, because of a trike's fairly wide stance, extra wheel, and their associated steering bits, they are slower, heavier, and more expensive than their 2 wheeled cousins. Leaning trikes can offer the handling of a two wheeler with the stability of a 3 wheeler. Tadpole trikes are better for handling at lower speed, but have a tendancy to fishtail at higher speeds. Delta trikes are better for high speeds, but don't handle corners are predictably as the tadpole designs. The Flevobike Versatile is a good example of this type of trike velomobile.

Bikes as chassis
A bike velomobile design is nice because it can be made fast and light. With this design you will need to either leave the bottom open or construct bomb-bay doors to put your feet down. The fairing can be narrower than a trike's and more aero. John Tetz's practical streamliners and the Lightning F-40 are excellent examples of this type of bike.

Suspension
While riding a fully faired vehicle on the street, especially at speeds over 20MPH, you are going to notice that it's hard to miss all road debris and imperfections. Also you will notice that a bike or trike handles a lot differently at this speed. Without adequate suspension, small pavement lips, and holes can cause pinch flats, and bumps can launch you. Full suspension is recommended for a practical vehicle for safety and comfort. Front suspension is necessary at a minimum. Fortunately the Pantour hub suspension now makes this easier.

Tires
For a practical faired HPV, wider tires like comp pools should be used. These will allow greater resistance to pinch flats and a more comfortable ride.

Fairing
A velomobile fairing should be designed with bike handling in mind, rather than pure speed. In general the bike will be shorter and more blunt than a speed racer, with curved sides and a relatively short tail. This will make the bike much less susceptible to side winds and gusts. It will also be easier to transport if necessary A velomobile should be shorter than 8 feet, as wide as your shoulders, and be tall enough to cover between your mouth and your bikes drivetrain. As with all fairing design, you should avoid flat areas, and make all curves fluid. Sharp edges like a V shaped keel or the angled joints in a Coroplast fairing which have little affect on straight line aerodynamics will cause the vehicle more susceptible to side winds. It's a good idea to close off areas like the foot well and head area that cannot be solid with some type of spandex material with slits for the feet and holes for the head. In a SWB vehicle generally you just need a large slit for the wheel, and then your feett can push the Spandex aside to allow you to put your feet down. Large open areas in the top or bottom of the fairing will not only slow you down, they will catch the side winds. 

Doors and Vision
With either design, much thought needs to be given to how the rider will enter and exit the vehicle. In general tops that flip open or large door work well. Also since the rider will be in traffic at least part of the time, it's very important that a rider has a 360 degree field of vision. This usually means a top-open design. Streamliners are generally noisy on the inside, and with your head inside the bike it's harder to hear cars and people yelling at you. Having your head out in the breeze is not a much of a problem in inclement weather as some would think, as the head opening can be quite small, which prevents water ingress. If your head gets wet but your body is dry you are still comfortable. A standard ski mask and goggles allow riding in the coldest of weather. 

Lighting
Practical vehicles are often used at night. Lighting should be considered in the design of the fairing. Also if the vehicle is to be used in high traffic area, turn signals and brake lights should be considered.

The Club Racer

The club racer is the jack of all streamliner trades. This vehicle is for a person more bent towards racing, who may ride the bike on the road occasionally, but will use it mostly for HPV racing. In general this bike needs to have the fastest fairing possible, and stick like glue in the corners.  Weight is not as much an issue with these bikes, and the fastest ones racing today are all in excess of 60 lbs. Lighter is of course better, but these machines are generally constructed with durability and crash survivability in mind.

It's a good idea to have a generous vent to blow air on the riders face, this will allow the rider to remain fresh during an extended race and prevent heat exhaustion (been there, it's no fun).

Chassis Considerations
Low racers with low bottom brackets can be used as a streamliner chassis. Most production low racers have a very high bottom brackets, which would place the feet too high in the fairing. Trikes are not a good choice for this type of vehicle. Another option is monocoque or "tub" construction. This involves building a very strong "tub" and integrating the front and rear sub frames. This is desirable as it allows the fairing to be shorter in height as the rider sits in the bottom of the bike. The rider seat back is adjusted to allow the rider to see over the knees (and fairing). Club racers should be in excess of 48 inches in wheelbase. Tires should be high performance and high pressure. Short or long wheelbase vehicles seem to work well for Club racers, with the slight edge to short wheelbase. RWD vehicles seem to handle better, but FWD can be faster due to more efficient rider packaging and chainline efficiency. Usually only two chainrings are needed, with a wide ratio. A very small one for drag racing and blasting away from the start line, and a mongo one for high speed sprints. A 23 tooth chainring difference can be handled by a standard long cage MTB derailleur. Use of wheel disks is recommended. Shorter riders may be able to get away with handlebars attached directly to the handlebar stem, but taller rider will need to use remote steering for leg clearance. Mustache or tiller handlebars will hit the sides of the fairing even in slight turns.

Trikes
As trikes generally cannot go fast around corners, and are hindered by the extra wheel and width, they are not recommended for club racers. If you must build a trike, note that tadpole (two wheels in front) trikes handle best, and that delta (2 in back) trikes handle at high speeds in a straight line better.

Suspension
You'd think that racing tracks would be smooth and that you would not need suspension for racing on them, but that's not the case. Most race tracks and velodromes are pretty rough, which will cause the unsuspended bike wheels to skip out and possible crash when cornering at high speeds, and to bounce over dips. Unsuspended streamliners have been know to "get air" when traveling at 40MPH over rough pavement. Many races can be won or lost in the corners. If you need to slow down to prevent you fairing from scraping or your wheel from hopping of bumps, someone else with a slower bike that handles better may win the race. Because of this, it's best to have at least a front suspension. 

Tires
In general, skinny racing tires work great and are aerodynamically efficient. Fat tires do handle better in corners, but the thinner tires such as the Schwalbe Stelvio seem to handle very nicely as well. 

Fairing
The fairing will be longer and narrower than a practical bike, typically between 9 and 10 feet. Width is again subject to shoulder width, but it should fit like a jacket!. Vehicle height should be as low as possible that still allows the vehicle to lean at least 45 to 50 degrees without scraping the fairing. Fairing scraping is one of the most frequent cause of streamliner crashes. Fairing height is from the lowest point of the chassis to the clearance need for the tops of the knees. Use of monocoque or "tub bike" construction will allow a lower fairing height, as the height of the frame, seat mounts and chains will not need to be part of the fairing height.  Unaided stopping and starting should be possible by the use of bomb bay doors or a landing gear. Fairings should be more pointy in front than a practical vehicle. Note that the pointier a fairing is, the more susceptible it is to cross winds. Even a moderately pointy fairing (like the Barracuda) is able to be handled without issues on windy days on the race track, but if you will be riding on the road more often, choose a more rounded nose design. Well rounded sides are not as susceptible to crosswinds as flat sides. As most crashes involve sustained slides on the vehicle side, and not hitting things, it's a good idea to incorporate a layer of Kevlar in the body shadow area. This material does not get eaten away rapidly while sliding on pavement. Other good materials for sliding are Coroplast, or multiple layers of fiberglass.  To allow a smaller frontal area and more gradual front end transition, a small pedal box should be used. This can be accomplished by using short, narrow Q cranks and narrowed bottom brackets. Cranks shorter that 155mm are not recommended as the power begins to drop off. Leave enough space in the hourglass shaped hole for the front wheel to allow the bike to make 90 degree turns on a normal 2 lane road.

Doors and Vision
With the club racer, you should be able to enter and exit the vehicle unaided, but you don't need to put your feet down. If you can open and close your canopy while moving and your bike is low enough for you to put a hand down, you may not need bomb bay door or a landing gear.  Note that you will often be required to sit for periods of time in the hot sun waiting for a race to start, so it is advantageous to have a top that can be closed and sealed quickly. Canopy tops are normally held on by hinges, located by tabs or pins, and held in place by Velcro or mini-bungees. Unless the bungees are very tight the top may vibrate annoyingly. DZUS Slide Latch fasteners have been used with good effect.  For this type of racer you will need good vision to the front and sides. This can usually be accomplished through the use of a flat wrap windshield, or blown plastic bubble. Rear view is required by rules, and can normally be accomplished through the use of small mirrors inside the canopy. Remote viewing through the use of cameras, periscopes, or mirrors is not recommended, as there is too much going on in a race to be seen safely in the narrow field of view afforded by these methods. Using a camera system for rear view is a great idea to see who's trying to hold onto your tail, as most streamliners have a blind spot directly behind them.

The Speedbike

The Speedbike is designed for all out speed, with compromises in all other departments.  These fairings should start out pointy (ok, not needle pointy, more like C-cup pointy), and gradually transitioning out to a maximum width at the shoulder area. This type of design keeps the airflow attached to the fairing for the majority of the length of the fairing which reduces drag. Length should be less than 10 feet to keep the "wetted" area to a minimum. Special care should be taken to keep the shape as monolithic as possible.

This vehicle should be as low to the ground as possible, but should still be able to lean at least 20 degrees to allow the vehicle to respond to side winds and gusts without crashing. Wheels should be faired or inside the fairing. Wheels disks should be used. These vehicles benefit from being completely sealed, but leave some small holes to allow the rider some fresh air to breathe. Some racers have sealed the wheels inside internal wheel fairings to reduce the air being pumped into the fairing by the spinning wheel.

Chassis Considerations
Speedbike chassis vary wildly. Monocoque chassis are recommended to keep fairing height to a minimum. FWD can be used without penalty to keep the drivetrain simple and lightweight. Super low Q and extra short cranks (down to 145mm) can be used to keep the foot box small. Seat angles can be fairly upright to allow efficient sprinting. Many racers have tried various forms of linear drive systems in the past, but none have worked as well as the traditional roundy-round cranks. Other system such as the K (Kingsbury) drive, which uses additional chains on the cranks to make the crank path more elliptical do work well to decrease the nose height, but add complexity and are hard to make very narrow. Speaking of things that don't work well, you can spend a lot of time just trying to get a rear steer vehicle to be rideable. Note that they get even squirrellier at higher speeds. Also, backward facing bikes are rideable but not conducive feelings of safety at high speeds. Prone bikes are good for sprinting, but are not good for longer distances.

Trikes
If you MUST build a trike, note that delta trikes work best for high speeds. Tadpole trikes can get squirrelly and start to wag their tail at high speeds. Make separate wheel fairings for the two rear wheels and place them at least a couple feet from the fairing sides to reduce the interaction between the main body and the wheel fairings.

Suspension
In general, suspension is not desirable on Speedbikes. High speed venues are generally smooth, and suspension can rob small amounts of power from sprinting efforts.  

Tires
Find the skinniest, lowest rolling resistance tires you can, and cram the biggest wheels in the bike that you can. The Varna speedbikes use dual 24 inch tires, but tire selection in that rim size is limited. 700C rear wheel is recommended. 

Fairing
The idea here is to cram the engine (uh... that's you) into as small a space as possible. Comfort is not a design criteria. The fairing should be narrow enough to fold your shoulders in, but not so narrow that you can't breathe. Fairing height is determined be the minimal space required between the knees at the top of the stroke and the riders butt plus anything that needs to got beneath it. Smaller riders have a clear advantage because they can be crammed into smaller fairings. Know any strong midgets? The fairing should be as smooth as possible to avoid any imperfections which would "trip" the boundary layer and cause the air to delaminate from the fairing surface. Bomb bay doors or landing gear are not needed as these vehicles are normally hand launched and caught by the racing team. Fairings should be sturdy enough to survive an extended slide, as a sudden gust, tire blowout or slippery road can cause loss of vertical hold. At the high speeds (60MPH+) that these vehicle are moving, riders need an extra margin of safety, so materials like Carbon Fiber and Kevlar should be used extensively. Very little room is needed to allow the front wheel to steer. Just enough to give the rider some "wiggle room" to wobble a little while getting started. Due to the very small amount of steering allowed, speedbikes crash most often while getting launched. Be sure that the tires cannot rub on the fairing, even under high side wind loads. This is the most common reason for high speed tire blowouts.

Doors and Vision
Any cuts in the fairings should be parallel to the direction of travel to prevent transitions that could trip the boundary layer. Because of this, speed bikes are usually split in two vertically or horizontally, and require team assistance to load and unload the rider. Speedbike vision needs to be good straight ahead, with some peripheral vision. Peripheral vision is needed to stay balanced when laying on your back, as the balancing mechanism in your ears don't work well in that position. Remote vision techniques such as micro-cameras with LCD screens or periscopes can be used, but it's recommended that small side windows be added to aid in the peripheral vision.

Resources:

Pressure distribution java applet
Drag of Streamlined and Blunt Bodies