The brushes and commutator are what you want to keep cool, brushes will stay coolest if they have good contact with strong springs and are seated in, same with the commutator, seating brushes in is best done by running the motor without a load until they are wore in, this can take days, there are short cuts you can take by using pumice stone, but I've been told by people who design motor parts not to take short cuts.
Forcing air in to the motor is a good idea, make sure it's clean air so you aren't getting grit in there.
I ended up buying a thermocouple reader and some thermocouples then drilled a 1/16" hole in one of my brushes and JB weld'ed the thermocouple in there, that way I get a reading from the most sensitive part of the motor I can, 300F is the point that brushes and commutators start to become damaged.
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