if your talking about yellow haze type effect dont bother wasting your time on polish alone, you will only waste time and effort with no results, and DONT use anything like neverdull metal polish or whateve people recomend this will put scratches in the headlight. The best and ONLY way IMO is simple..... go to a body shop and ask them if they will sell you some sandpaper. Im talking about the really good wetsanding kind. Get 1200 1000 and 800 grit. Wet sand them down in stages (obviously start with the rougher grit) and then after you think you got it all just use turtle wax polish to restore them..... If you do this it will make them look just like new again. Its what i did and i couldnt be more pleased with the results. The look as good as the day i got the car.