I'm a big fan of 3M's headlight renewal system-- they come in three flavors, basic, plus (includes high quality masking tape) and top-of-the-line (includes masking tape and 3M sealant for a final coat).
This one was the "plus" edition which had the masking tape which is a necessity unless you have an older car or you don't care about leaving scratches on the surrounding bumper/fascia. 3M's kit takes a while-- it took probably an hour each side, and I haven't even sprayed down the mess it made around the headlights (which you can see in the pics.) The time spent is well worth it, the results are pretty amazing every time. I get about 18 months before the plastic starts to cloud and yellow again, requiring reapplication. Not bad for a $20 kit. I'm surprised this is not a universal pre-sale procedure at used car lots- it adds huge value to a used car IMO.
This is a used 2007 Chrysler Pacifica Limited with 129k miles I purchased recently for $5k. The headlights were yellow, and the motor mounts were shot when I bought it. I was able to negotiate the price down a fair bit because of these two things. A couple afternoons and both are fixed. This particular car has the $595 (when new) Xenon HID option which is only available on Limited models-- The 3M kit did a fantastic job, I topped it off with some good synthetic wax, but in the future I'll spring for the version that includes 3M's sealer.
The spatter from the rubbing compound/polish look like scratches in the photos, but it'll wash right off.