[Rickertsen2]

The above mentioned encoders are probably best, however if you really want to use a potentiometer, there is a way to modify pots for continuious rotation... well sortof (there will be a region of several degrees where the resistance will be an open circuit.)

But anyway here's how:
1)carefully bend back the tabs whichhold the metal back on.
2)Directly opposite the point where the wiper contacts the circuit board, there is a little part that protrudes out further thna the rest of the platic disc. Cut it off. Alternatively you can bend out the little indention in the metal cover.
3)put the thing back together.

I do not recomend this.