A product which I have had great luck with is actually a motorcycle product made by S100. It is called "Engine Brightener". It is a non waxy, non sticky Armor-all like product, which dries quickly and lasts much longer than Armor-all. It also feeds and rejuvenates plastics.
This has been a secret of gun dealers who use it on polymer guns, especially used ones to make them appear factory new. It works on the metal slides as well.
http://www.s100.com/s100_eb.htm