Depends on the amount of hazing/scratches/yellowing on the headlight lenses. Some have to be wet sanded, some need only a plastic polish, some need a heavier polish.
I seal them after the final polish. I've used Collinite 845, Krylon UV Gloss Spray, NXT 2.0, PlastX. I have some Wolfgang sealant from the Geek kit I'll try also.
I also got my hands on a coating that seems very promising, but I haven't tried it yet. Claims: Durable, pliable, will not crack, chip, peel, or yellow. Waterproof, inhibits growth of mold/mildew. Cured coat can be cleaned with mild detergent & water. If applied to a high-traffic area and wear begins, wipe a new coat over the wear area.