The most common way to burn up a motor is to not throttle down/off
when the helicopter hits the ground.
Leaving the motor spinning when the blades can't turn burns it up.
If the rubber bands that hold the battery up are rubbing the main gear
that will cause excess stress on the motor..
If the motor is pushed up against the main gear too tight that will cause
excess stress on the motor... and when you replace the motor make sure
the screws that hold the motor on are not rubbing against the main gear stressing the motor.

There should be just a tiny bit of wiggle room between the motor gear and the main gear.