Having worked in and out of Body Shops all my life I have seen a variety of processes used. The best is a professional kit produced by 3M which the shops usually charge anywhere from $39.95 to $69.95 and it does look almost as good as new.
However depending on the vehicle age, you can actually purchase either After Market or Re-manufactured Head lamps from your local parts store or on line for about the same price. You can also purchase a repair kit now for about $9.95 at most Walgreen and Wal-mart stores which works fine depending on the age of the vehicle. Hope this helps.