CO2 car racing encompasses many aspects of engineering, not the least of which is aerodynamics. Since most CO2 cars in a given class weigh about the same and have the same amount of thrust, making the body as sleek as possible can give you a real edge over the competition on race day. Some of these aero tricks can be applied to an existing car while others may require an entirely new chassis.
Covering the surface of your car with small, semi-circular pits ("golf balling") does two things. First, it helps to shave weight by removing material. Secondly, it can help minimize drag by reducing the size of the drag vacuum pocket behind the car. These pits help air to form a very narrow and slippery boundary layer over the surface of your car instead of dragging out behind it and creating a vacuum pocket. This idea is far more than mere fodder for bench racing; that a dimpled golf ball will travel almost twice as far as a smooth ball is well documented. Discovery Channel's Mythbusters once tested this principle on a full-sized car and recorded an average increase of 3 MPG in fuel efficiency as a direct result of reduced drag.
The bottom of your car experiences just as much drag as the top, so make sure to keep things smooth and straight all the way around. The ideal car underbody should be completely flat, parallel with the ground and as close to it as possible. Keeping your car's underbody as low as 1 millimeter (even if your class rules dictate a minimum height) can nearly eliminate airflow under the chassis for minimum drag and maximum speed.
Sleek Over Small
Given that your particular class has a minimum weight requirement (as most do), you'll be far better served by making your car sleek rather than making it compact. For the purposes of discussion, we'll define "sleek" as a minimum angle from the front of the car to its highest point for a wedge-like shape and absolutely zero protrusions when possible. Although you may be tempted to reduce overall frontal area (the size when viewed from the front) by using a separate fuselage and wheel "pods," don't do it. The "pods" themselves are fairly aerodynamic, but the mating point between them and the axle are a breeding ground for high pressure air, and the area behind them will provide yet more opportunity for drag. Instead, focus on building a "shell" type car, where the entire drag pocket is focused in a single area behind the car. It's a lot easier to reduce the size of one big drag pocket than it is five smaller ones.