by Hib Halverson:

Since plastic headlights and headlight lenses have been around for decades, the aftermarket has responded to the UV degradation problem with headlight repair or restoration kits. We've looked at some and the one we prefer is Sylvania's "Headlight Restoration Kit". This product is available from a wide variety of sources and, not only did it work well in our testing, it is highly-rated by Consumer Reports.
Product Review: Sylvania Headlight Restoration Kit | Cadillac V-Net