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.