Here's what we did at the garage. We charged $40 a car, and had it down to a science.
3 products:
Mother's professional head light restorer kit
"Lik Nu" plastic window cleaner (Stuff to clean jeep windows)
Rain-X.
Moisten polish ball, apply polishing compount to ball, smear polish on head lamp. Polish head lamp at very low speed, apply tap water via spray bottle to head lamp, keep polish "slurry" to the consistancy of milk. Continue to buff until flakes, scratches, and yellowing is removed. Allow polish to dry. Buff off remaining polish with clean rag. Apply Lik Nu as directed, this will pull the extra polish out of the small scratches, and further brighten the lens. Skip this step and the lens will look like crap. It will have the appearance of someone sanded it with 1000 grit paper. Apply Rain-X as directed to seal lens. Skit this step, the lens will yellow in about a month.
Never apply any waxes to the lens. Waxes will etch the plastic.
The idea is, don't let the lens get too gunked up. If you think you need a polish job, do it immediatly. If a lens gets too bad, there's no way to save it. Yer stuck replacing it....and those are really expensive. Also, it's a continual maintenace thing. The discoloring will come back. You will have to have it polised again.