I'll be interested to know the answer to this myself, since the headlight assemblies on my '03 Corolla are getting that typical yellowing that is common to the outer plastic shell. I wish they were made of tempered glass instead, but I'm sure that would be prohibitively expensive.
the yellowing is pretty easy to get off. they sell kits that allow you to restore the surface of your headlights. they work pretty well. they consist of different grades of sand paper (you sand paper the headlights with progressively finer sandpaper) and there's also a refinisher you apply to the plastic surface of the headlight.
Hmmm -- I've always wondered if such a kit were available. Do you know what it's called and where to get it?
There's a bunch of different kits out there but they all contain the same basic components (different grades of sandpaper and a refinisher). I just went into my local Autozone looking for it and they had a kit, it cost in the neighborhood of $20-30. I saw a similar kit at Walmart once.
And if you follow the steps properly and thoroughly, the results should last. I refinished the headlights on my '99 Prizm a year and a half ago and they headlights are still clear.
BTW, most of the kits I've seen don't require a drill. The kits I've seen use handheld sandpaper.....it works fine. I sandpapered my Prizm's headlights by hand.