I've tried a number of things, including a couple of those $20 a pop kits. But the very best I've found is a pack of 2,000 grit wet or dry sandpaper (for if it's really ugly) and an 8 ounce bottle of "BlueMagic Headlight Lens Restorer," which I got at Advance Auto 3 or 4 years ago for like $12. That 8 ounce bottle is like a lifetime supply, as it takes maybe a teaspoonful to do both lenses, and it works like a charm. Of course, you do have to redo it every 6 months or so, but it only takes about 5 minutes per lens. When I get done, the lenses look brand new.