Originally posted by MrFatBooty
Is it just the outside? If so, get some Mothers Plastic Polish. Should be able to find it at the local auto parts store, Wal-Mart, wherever. Pop out the headlight housings to get at the whole lens, just make sure you realign em when you put it back in.
For extreme cases (80's era cars) I've also heard of people sand blasting, but that eventually destroys the headlights.
Shingo