Originally Posted by Mike-in-Orange

You can clay any surface that is hard and glossy - paint, glass, headlights, tail lights, glossy plastic trim, etc. You just want to stay off anything with a matte or textured finish like the black plastic cowl at the base of the windshield, textured black plastic side mirrors and stuff like that.

Generally a quick detail spray is the best choice as a clay lube and a bottle is included in both the Mother's and Meguiar's consumer detailing kits. A few drops of car wash soap in a spray bottle of plain water can work too, but some clays don't like the soap and will break down fairly quickly when used this way. Plus, using soapy water generally means you need to rewash the car after claying it, something you don't need to do when using a quick detail spray as the lube.