Yep,
Slipper clutch would be the first this. You should have a foot or two of slip.
(And feed in the power.)
What motor are you running?
Other thing would be to add weight to the front of the car, around the bulkhead or around (under) the servo.
Ghea make a brass bulkhead for adding weight to the front of the car.
(Business plug, these are in stock at
http://www.p-dubracing.co.uk)
Or you can just use any sort of weight on the front of the car. Allow wheel balancing weights are good for this.
MiCk B. :-)