I think guys have actually wet sanded them before. Also, there is a plastic polish for visors and windscreens that would probably do a world of good. I forget the brand name but it is sold at most bike shops and comes in coarse, medium, and fine. Also peanut oil and/or lemon oil. And finally Mothers Back to Black works well.
I actually used Mother's Back to Black and it works beautifully where the gloss still remains- but on the areas that are well-worn, not so much.