Body kits only work if they are wind-tunnel designed and scrape the ground. Such kits would be shaved off by road obstacles in a week on a daily driver.
Most common fiberglass kits do nothing but add weight and drag. They're ornate, have flaps, vents and all manner of turbulence-inducing addenda. A smooth body saves fuel. If it didn't, the Prius would be sporting one b*tchin' set of skirts... :lol:
You can try various smaller aerodynamic devices. Vortex generators (work on trucks, but on cars, it's a big ???), underbody flaps (in front of the wheels, to push air away from the wheel-wells, which act as big parachutes) and plates to block off the rear bumper area all help to some extent.
You can use Vortex Generators at any edge that will affect airflow over the rest of the body... the wing mirrors, the corner at the headlights, etcetera... you can check to see their effect by taping small streamers (or whatever they're called) to the side of the car and observing how wind flows over it at speed both with and without the vortex generators. The rear edge of the roof is also a good place to work at... Mitsubishi placed the Evo's generators here to guide air down to the wing, you're doing it to push air away from the trunk.
As for underbody work, you might need a pressure meter to check it... or you could do the mythbuster thing and build a fuel-flow meter to check how your modifications are affecting your gas consumption at constant throttle and speed. One surprising result of that Mythbusters episode was the effect of a net on a pick-ups fuel economy... having an open bed and a net on a common single cab was actually more effective than having a cab cover. I suppose it has to do with vortex generation and flow, but then, whatever works, works.
