A few I helped my son when 2nd place in his pack a couple of years ago were:
Polish the axles on the lathe. Make sure to ream the seams out of the insides of the wheels. Also the axles (nails) have flashing on the backside get rid of this and polish them up real nice. Also remove the flashing on the outside diameter of the wheels.
Do not use the pre-made axle holes/channels. Drill them yourself, make sure they are all straight and stuff. If you use a mill, it's really easy to set up the 3 of the axles holding the car up, and the fourth not dragging the car down. Balance it on 3 wheels
Weight as far back in the car as possible (greatest gravitational influence, in the front once the front of the car gets to the flat gravity's done, with the weight back even when the front of the car gets to the flat the weight is still pushing)
Wear in the wheels on the belt sander with copious amounts of dry graphite.
Work the insides of the wheels where the get up against the car, polish this up really good, and make sure the wheels are up against an area without paint. Use the graphite here in excess.
The more accurately you can weigh it the closer you can get it to the heaviest you can run. If you're lucky they will have a weigh in early, so you can check their scale against yours.
Those are the major areas that will put it in the running.