A Cesna pilot showed me a great way to make the plastic windows clear again. We went to one of the little specialty stores in a small airport and got some Meguiar's Mirror Glaze. They use it to clear up plexiglass airplane windows. (It's not just for airplanes, so it might be available at auto supply shops too.) It's 2-step--first clean then polish. You put a buffer on an electric drill and rub that stuff around aggressively. Mine was so bad, I had nothing to lose, so I really went at it. On an 18 year old top, it wound up looking just about new. I think it would have looked completely new if I'd unbolted the top frame to get to the inside better. Eventually, I had to replace the top, but the windows still looked great.

The stuff didn't leave any residue to speak of.