Honestly, I found the best thing was to replace them and get the custom cut headlight protecting film for them to protect them from the UV and sandblasting effects of sand, dirt and stone chips while you drive. The best protectant if you do not wish to do that is the 303 spray. Much better than anything else for protecting plastic, vinyl and rubber.
If you do a search for the 303 homepage you can order a free sample of the product and their brochures for their other stuff.
