Some people thin with water, some thin with windex or windshield washer
fluid. Some use isoprophyl alcohol and use a acrylic retarder such as
Liquitex or Golden (make sure it's the thin kind!). I use Tamiya acrylics and Tamiya
thinner, but I buy the 250ml bottle, not those dinky little bottles! The Tamiya thinner comes with a retarder already added.
A great source for acrylic paints and airbrushing (since MikeV doesn't have a website
) is
MusicCity's website. Scroll down for airbrushing tips, thinning ratios and more acrylic paint references than you can shake a paint brush at!