there's many threads on this topic, with all kinds of details....use the search bar and try words headlight cleaning, or Maguiar's headlight cleaning, 3M headlight....many many of us have done it with great success. I used the 3M kit.
Only downside is you are sanding off what little UV protection they may have had.
BUT....there's a liquid spray on product by Blue Magic that is a UV agent....and some others have had the entire lens (or entire front end) coating in a clear coat material with a UV protectant. VT_Maverick and Rowhard have both done it.
I'll start ya out with the thread I started on the 3M kit:
http://www.vehicross.info/forums/sho...ht+restoration