Use 1500 and then 2000 grit sandpaper to wet sand them then follow up with some Meguiars Plast-X plastic polish. I've also heard you can use non-acetone nail polish remover to remove the yellow, but I'd be careful as not to drip the nail polish remover on your bumper. I'd just go with the wet sand and plastic polish. I've done it and they come out GREAT. The only problem is that there is a clearcoat on your headlights (this is what yellows) and after you wet-sand them, you remove this clearcoat, and you will need to continue wet-sanding them every now and then because they will get a little cloudy after a while UNLESS after you wet-sand and polish them, you cover them with Lamin-X. (Google it)