I spray my belts with Silicone spray, on the inside. Then I wipe off excess silicone around the belt. Be sure to never use Silicone spray near or even in the same room as an open engine/air filter as when it comes into the engine, can cause a lot of problems.
I know that some other team drivers shave down their belts. This makes the drivetrain even freer. You always want as free a car as possible to save runtime, and increase acceleration.
ORCAN offer Silicone spray:
http://www.orcan.de/index_prokat_pflege.php