Hello Justin,
Well, I'll start with my paints. I'm using very very thick acrylics just because I'm also painting with painting brushes. They're most opaque paints that I've ever used in my life, and very efficient. Without thinning down you can paint almost 30 square meters just with one liter of paint. That's much. But to the point of this topic. In my 18 months adventure with an airbrush I've used a lot of different reducers. Some were better than others, and some were worse. And here's what I've found- best reducer for acrylics ever is... vodka

Really. Cheapest, worst 40% vodka. It's thinner than water and weighs less, so your piant will flow better. I've heard (and of course tested it) that cleaning solutions for glass (like Windex or Mr. Muscle etc.) are also good. Yes, they're good but you need to be carefull with that and airbrush, 'couse it produses a lot of spume so you can damage your work with that. With regular paint brushes is as good as vodka.
And here are my reducing rations with paints I'm using (by weight)- 2 parts of paint and 3 parts of vodka with Royal Talens Amsterdam paints, and 50/50 with Maimeri Idea Decor paints.
This way you can thin down any type of paint, just with some patience. Simply get some paint into container and add small ratios of reducer (vodka) to it. Look what has happened, and add more if it's still thick. Useful tool for that is an electronic scale

Using it you'll be able to make identical paint over and over.
Another good idea is to get a piece (I have about 15 cm x 20 cm) of transparent plexiglass. Reduce your paint, put a few drops on plexi and look how its running down. If it's slow, reduce. If it's too fast, add more paint. Coverage of paint should be even and semi transparent or opaque (if tou're using opaque paints) and it's running down should be smooth and even.
Sorry for my English, hope you understand all of that crap
Regards,
V
P.S.
Here's a video I've found about 2 moths ago so somebody just made it faster than I did ;p BUT! Isopropyl alcohol is not a good idea. It's toxic... Belive that vodka is best.
http://www.youtube.com/watch?v=s6LVub0j_3Y