maybe you could try to make the underside of the chassis as flat as possible, creating less drag for air under the chassis. if you can speed-up the air flowing under the chassis, the difference with airspeed flowing over the body is minimized, resulting in less lift.

edit:
some additional info

Quote:

Even with these limitations, the underbody is of major importance in the aerodynamic setup of the car. This is particularly true on the speedway circuits. In order to achieve an efficient setup, the front and rear wings are much smaller than on a road or street setup. The frontal area of the car is reduced in size so it can slice through the air, in an effort to reduce the drag coefficient. The front wings are not used for downforce on the speedway setup, but rather as a trimming device. The major downforce comes from the chassis and underbody design. The rear wings are also much different in appearance on the speedway setup. Instead of the standard wing or high downforce wing used on the road and street setup, the rear wing is almost horizontal with smaller endplates. The speedway rear wing is shaped to create less downforce and reduce drag. The optimum setup on the speedway is to create enough downforce on the cars so when they reach the corners they can run flat out. Drag reduction is more important than downforce on the speedway setup. The best setup would be an adjustable wing to reduce drag on the straights, but supply downforce in the corners. CART regulations, however, prohibit this type of setup for safety reasons.

found here: http://www.nas.nasa.gov/About/Educat...r/chassis.html

or, add a fan at the rear of the chassis: http://www.autoevolution.com/news/gr...17.html#image2