I've heard of guys using a vice and then a high torque drill to tighten the winds slightly to get 20-23,000 rpms at 7.2volts. other ones I've heard are using a Dremel to break in the motor rather than a battery, you tighten the Dremel to the armature and then spin it in a glass of water at max speed for a couple seconds (usually 30,000rpms) and the motor will then spin 25,000 at 7.2.
Another trick I've actually seen is someone put a silver-can into their magnet zapper before a race. Actually works well for the first three minutes or so if you can manage to drive a clean line you can try and put laps on everyone until the zap wears down.
PS: Silver cans come as Mubashi OR Johnson. Depends on the model and where it comes from.