70% isopropyl alcohol (IPA) (rubbing alcohol) that you get at the drug store is an excellent thinning medium. Probably the best is the thinner recommended by the manufacturer, although I think you wouldn't find much difference between their thinner and the IPA. Typically, manufacturers thinners also contain some gycol ethers that also offer some additional stability.
You can thin with water, but if you do, add 1 SMALL drop of dishwashing liquid to your water. Water has a natural surface tension (look at water on a freshly waxed vehicle), and has the tendency to do the same thing on styrene when you paint. The dishwashing liquid will help break down the surface tension, and give you a smooth finish.
One other option is to use the methanol-based windshield washer fluids available at Wally World--oops!--Wal Mart or other auto parts/hardware stores. Methanol has many of the same characteristics as IPA, and should provide a pretty good finish.
Experiment and practice to find what's best for you.
Gip Winecoff