One source of dirt I haven't seen mentioned is motor dust. It comes from the carbon brushes inside the motors. Have you ever noticed how dirty Addams gets? Thank Thing for that.
I agree with Cody about metal ball guides. I used to get RePlay magazine. They did an experiment with a pair of TZs, replacing the stainless steel return lane guides with custom Lexan ones on one of them. The difference was huge - the game with the plastic guides was much cleaner.
Excellent thread, carry on!