[t-squared]

To get more torque and lower RPM and horsepower, put more turns of wire on the armature. Your motor will then draw fewer Amps, and therefore produce less total horsepower. A motor with more turns will develop more torque per Amp, but it will draw fewer Amps, so it is hard to predict exactly how much torque or RPM it will develop after the change.

To get more horsepower you should use fewer turns of a larger gauge wire. This will reduce the terminal resistance of the motor, and increase the RPM. The motor will develop more horsepower, but it will be at a less manageable RPM.

http://www.robotbooks.com/motor-tips.htm