are they plastic? assuming they are, a good plastic cleaner and then polish such as Meguiars PlastX will make a world of difference. Its going to take some elbow grease but should turn out pretty good as long as they arent too far gone. Also after you get them that way, try and track down some of that 3M clear film that they put on headlights and get some for your car, that should also help keep them looking new.

hope that helps