What you need is something like this. All kinds to choose from that are rated at various amperage and voltages, with or without reversing switch. Ive got a few of them and they work great and produce very little heat compared to a variable carbon pile resistor.
http://www.ebay.ca/itm/PWM-Motor-Spe...item33afe99d6a
http://youtu.be/o1rKK4gUuz4