There are several different methods that I've heard of.
Water is the first. I didn't add water until I was using the paint as a wash, but IIRC the general thought was 1-2 drops of water per 4-5 of paint. Start at the 1-5 end, and add more water if necessary.
There's also a flow improver/medium that you might be able to find (also from Apple Barrel/Plaid). I haven't seen it in years, but I do remember seeing it.
I've also heard of people using Future floor polish to thin the paint a bit as well. I don't know if the new stuff (Pledge with Future, or some such) works or not. If possible, I'd stick with water or the flow improver.
Do you read Sutter Cane?