Head Light Cleaning Rule of Thumb.

There are three basic tools used to restore clarity to an acrylic headlight lens.

Sandpaper = total resurface = massive abrasian = when all else fails

Polish = light abrasive and damaging to lens surface = safer than sandpaper

Acrylic Deoxidizer = Non abrasive, no damage to lens surface, UV layer safe = safest method of all

More of a break down on when to use which:
http://www.clublexus.com/forums/automotive-care-and-detailing/641035-headlight-cleaning-101-a.html