My son was started cub scouts when he was in the first grade and stuck with it until he was awarded his Eagle Scout at 17 years old. One of his favorite memories is pinewood derby! Here's a few tips and tricks that I remember from the five years we took part (some of the tips were mentioned previously in this thread.)

During the manufacturing process of the plastic wheels there sometimes is flashing or small sprue's left from the injection mold. We would ever so slightly sand any of that off the wheel and also knock down the sharp edges of the wheels ever so slightly. Be careful when you do this as you don't want to alter the wheels so much that they appear modified and the car gets disqualified.

As Steve said make sure you polish the nails. The way we always did it was to place the nails in a drill motor and then use emery cloth and finally cotton cloth to polish the axles while the nail was slowly spinning. After polishing the nails we would remove the nail, put the wheel on it, and chuck the nail back up in the drill motor. We would then put copious amounts of graphite powder into the hole in the middle of the wheel and on the exterior of the wheel while spinning the nail. This seemed to impregnate graphite into the plastic where the axle makes contact with it.

Hi cub scout pack always had a weigh in the night before the race and after that the cars were "impounded" and nothing more could be done to the car including adding graphite. So before my son weighed his car I had him put additional copious amounts of graphite on the axles and wheels. After that I would have him put more graphite on the axles and wheels. During the time between getting his car from the impound table and placing it on the track I coached him to NOT spin the wheels, which is a natural thing to do for a boy to do. Our boys always raced multiple heats and spinning the wheels causes the car to lose graphite and slow down. Also I coached him to hold onto his car with BOTH HANDS as there have been numerous times where a kid was squirreling around with there car and dropped it; race over!

As others have stated they lightened their car and put lead weight in the bottom to get the car back up to the 5 oz. weight limit. If you use molten lead make sure you make the bottom of the hole you're going to pour it in wider than the opening of the hole so when the lead cools off it is wedged in the hole and will not move. I have seen countless cars have their weights come loose and cause them to drag on the track and slow the car down.

After you have the wheels mounted on the car set it on a flat surface and see how the wheels look. A lot of times you will find that only three of the four wheels were actually touching and one was not. The intuitive thing to do would be to adjust the car until all four wheels were touching the surface. We were always convinced that a car with only three wheels touching the track was FASTER than a car with all four touching the track due to less friction.

Finally, the most important thing of all is for you and your son to HAVE FUN; what an awesome program Boy Scouts is! After the race is over please let us all know how he did!

Regards,
Bill