A rotary encoder! Now that's an elegant solution!
The tool JoyToKey is helpful as well, since it's generic it will work on any device that's recognized as a game controller. Currently I only use it on my stick, buttons 3 & 4, to zoom in & out in a smooth fashion (button 3 makes JoyToKey push the I on keyboard 25 times a second, button 4 the same with the O on keyboard).
Using multi-turn pots is beauftiful but I imagine those things being expensive. You might consider using normal pots and use a non-linear scale so movements around the center doesn't result in sharp changes. I use cheap Saitek 3 lever slides, one for power/proppitch/flaps, the other one for elevator/rudder/aileron trim, which works fine with default settings for me.
http://www.fspilotshop.com/images/saitekusaP45.gif