What type of motor is it? If it is a sealed can (non rebuildable motor) it is very hard to clean the comm & brushes. Motor spray will help, but if the brushes are burnt, the motor is pretty much stuffed.

If it is a rebuildable motor, give it a good clean with motor spray and wipe done the comm and brushes. If the brushes are heavily tarnished, it is time for new brushes.
If you pull the motor apart, there are usually shims and an oil sheild, so just take note of where they go.

Just remember that motors need a chance to cool down in between battery packs as they get very hot. After five minutes of racing, if the motor is very hot to touch (should be able to hold you finger on the motor for a few seconds), it is over geared.

If in doubt, go down to your local hobby shop or from where you purchased it.