a lot of places sell a plastic cleaner, with the logo "PLASTIC CLEANER" on the tubes for about $20
You rub it on, leave it on for about 2 - 5 minutes, then thoroughly wipe it off (you could use some form of buffing device if you wanted) and they come up VERY CLEAR.
I've done it on my S2 (which is definately plastic) and they look CRYSTAL clear (especially with the xenons I just put in them)
A mate of mine has a Series 1, and we used it on his headlights and they came out just as good.
Jason.