I made it easier to clean the lenses on our cars (Neons, Escorts, etc.) by taking the headlamp assembly over to my bench grinder, and grinding off those 3 little pips used for aiming at the factory. Then its easier to do the sanding/polishing. I use Chrome Cleaner as my polish.
I was in a hurry to get the lens -polished- one time, and used my propane torch to 'melt' the plastic to clarity. I moved the torch in a sweeping motion, going slower each pass, until I learned how fast I could move it and not ruin the plastic. The headlamps were out of the car for this. This is a fast way to bring the lens to clarity once you have sanded all of the yellowish coating away, but..... Dont blame me if you set your headlamps on fire. And it may cause some crazing of the plastic to appear.
On another forum a gent mentioned a Fast way to clean up really fogged lenses. He sprayed Carburetor Cleaner on a paper towel, and gently ran the paper towel down across the lens. I tried this on the worst headlamps I could find - a neighbors Toyota. It worked wonders, but its easy to overdo it. The Carb cleaner actually dissolves the surface of the lens, taking the yellowish crud with it. This will be obvious by looking at the paper towel. It takes a few hours for the lenses to fully harden after this "Carb Cleaner' treatment, so give it a day if you want to polish them further.