Always start with the motor whatever make, use a can slug to ensure it is square, ballrace in can, align brushes, shim, glue in and hone the magnets. Run the motor until warm and true the comm while still warm. Reassemble the motor with minimal armature float and shunt the brushes and run the motor in. Other simple things to do are , ensure the chassis is flat, fit axle ballraces, ensure the guide doesn't wobble about and use good quality soft braids. If you want to take it to extremes fit hollow drill blank axles. Could go on forever and be a real bore but I'll leave it at that.
Tony
Tony