I've had success with Mother's headlight restore kit. It comes with fine grit sand paper for the extremely bad lights, a power ball with polish and a micro fibre rag for finishing.
Works real good too. I think the kit was something like $20. They also sell a sealant to help keep the lights clear longer.
I've used it on all the plastics on my vehicles. Turn signals tail lights etc. Makes it all look new again.