I have been using potentiometers (or rheostats) for controlling fans in my case for nearly two years. The biggest challenge I encountered was finding a source for these potentiometers (pots) that would handle at least 5 watts without breaking the bank. One of the best sources I have found is Digi-ke...

Digitaldoc 5 with the flat sensor (plastic shaved so it is just covering the edge of the wire) with the contact touching the side of the core, copper shim with a slot ground so the wire will route without causing the heatsink to lose contact with the processor core.