There are quite a few products around that can be used to restore your headlight lens covers.
There's - Meguiars Plast RX Clear Plastic Cleaner and Polish
And - Xerapol Plastic Polish and Acrylic Scratch Removal Kit
And - Novus Plastic Polish (a pack of 3 grades of 'restorer')
video here:
http://www.youtube.com/watch?v=9qvGnFecVDs (see from 5:16)
And possibly many many more, but I would recommend sticking to the products of the most
well known companies in vehicle detailing.
BE AWARE.....Nearly every headlight lens cover is treated with an anti UV protector. Use of any of the above
will remove/strip this layer off, so you must use the polish recommended, or any good quality
polish to replace what you have removed. And remember, you must re-apply this every time
you clean the car, or your headlight covers will go hazy very very quickly !