This is taken from a post on the Armorama forums, dealing specifically with Polly Scale acrylics. The post is from someone who has been around for quite a while in the modeling community:
For Tamiya acrylics, Polly Scale acrylics, and Model Master Acryl, thin with $1 a gallon windshield washer fluid. All of these paints use isopropanol, glycol, and water as their carriers. Windshield fluid is isopropanol, glycol, water, and detergent. The detergent acts as a wetting agent for the paint, breaking the surface tension of the paint and making it flow and self-level. This is exactly what Tamiya thinner is made of, but Tamiya thinner is $128 a gallon! Don't worry about the blue tint, it doesn't show up in the paint, even white.
Vallejo acrylics and Plaid acrylic craft paint must be thinned with distilled water. If it dries too fast and clogs the airbrush, go to an art store and buy a bottle of acrylic medium extender. It lengthens the drying time so the paint won't clog or pebble. Extender (also called retarder) can be used in any of these paints.
If you want to find out what any model building substance is made of, if it's sold in the US it has an MSDS (Material Safety Data Sheet). This will tell you whats in it, and therefore what to thin it with. Googling the product will direct you to the MSDS for it. Many times the carrier medium is right on the label of the product.
Start with thinning 50/50 and adjust from there. Different brands need different ratios. Even different colors within the same brand will need different ratios, due to the different pigments used.
As you get closer with the airbrush for tighter patterns, you need to go thinner on the paint and less on the pressure. Paint will accumulate on the needle/nozzle as you airbrush for long periods. Keep a cup of Windex and a Q-Tip handy to clean the needle/nozzle as you go. If it clogs or spits, you need to clean the needle/nozzle.
Finally, you don't need to take apart your airbrush every time you change colors or use it. That's just asking for trouble, a bent needle, and diminished performance. Between colors and when you're done, wipe out the cup and shoot Windex through the airbrush until clear. Make sure the nozzle is clean. Put a drop of Super-Lube in the cup, and hang it up. Done.
I can attest to the windshield washer fluid thing. Don't ask me why, but it works for all three (I have all three types that I use without fail) brands of paint. I use an Iwata HP-CS and use the little Badger hand held paint mixer, then strain it through a small patch of stocking (pantyhose, just don't tell the wife...actually DO tell her, then she won't look at me all crazy when I ask for her old hose...but I digress). The mesh filters out crap and also the air bubbles, and I can FINALLY get a 1mm fine line after a week or so of splotching, running and clogging tips.
Just my but it definitely helped me out a great deal.
Here is the original post on Armorama:
http://armorama.kitmaker.net/modules.php?op=modload&name=SquawkBox&file=index&req=viewtopic&topic_id=116146&page=1