If the Meguiar's plastic polish doesn't do the trick you can try auto body rubbing compound, I don't know if I would try to cheap Turtle Wax stuff, but once again the Meguiar's product work well, I have used it on windows before and you can do like you would with rough paint, going from more aggressive to finer polishing and finishing with the Meguiar's plastic polish, they also make a plastic "cleaner" which is actually a more aggressive polish than the polish, the polish is really mild.