Between heats at the races it is a good idea to spray out your motor with motor cleaner - let the motor cool somewhat first. Lube the bushings (or bearings if not stock class) with a drop of oil and go race. If you are racing on-road then cleaning so often probably isn't a big deal but definately do it after each heat if racing off-road.
Between racing days I will pull the endbell and clean the comm and brush ends AND SIDES with a Trinity fiberglass bristle comm pen. That little pen works wonders between comm cuts. Every 3 or 4 race days I'll cut the comm and put in new or re-serrated brushes.
You can cut comm and replace brushes more or often depending on how much time you want to spend and how long you want your motor to last. I just race at the club level so I don't get too carried away with it.
Don't forget to take care of the batteries too.