A good UV protective clearcoat is the best option, at the paintshop i buy auto paint at theres actually spray cans of Dupont Chromaclear, you will need to clean the headlamp and wet sand with 2000 grit.
Ok . . . you HAD to make me go way back into the search archives. This is one of my fave's. This dude had WAY too much time to work his headlights. But think of the possibilities? Wouldn't it be COOL to spray a little smoke tint in there? It would also be extra slick if you could run a row of teeny LED's accross the center . enjoy http://priuschat.com/forums/gen-ii-prius-technical-discussion/52247-how-bake-headlight.html .