are you talking about fog building up behind the lens so it looks hazy?
if so that means your headlight assembly isnt air-tight like its supposed to be. its a pretty easy fix - you hafta take out the whole headlight assembly, check the o-rings around the light housings, and check to see that the lens is sealed to the rest of it. i used a little silicone gasket sealer to make sure it was leak proof.
if you're lost by the above techno-babble, or thats not the problem you're talking about, give us some more details so we can figure it out (or just tell me and ill explain what i said in newbie-terms).
If it's on the outside you might want to try a plastic polish like those sold at Boat USA or autosupply stores. I find that the marine stores carry it more often due to many boat owners having plastic windows.
The stuff works extremly well and kept my Miata's plastic rear window crystal clear for years and years even though it never saw a garage and was primarly my winter beater and saw tons of salt spray.
I just replaced my drivers side H1 headlight low beam bulb and found upon inspection of the burnt out bulb that it slowly produces a sulphurous gas inside over time making the headlight appear yellow. I turned my lights on at a glass window and saw that the still working passenger bulb was yellow compared to the newly installed drivers side one which appeared very white, so I switched out that bulb as well. This may be part of your problem as well.