All I can add it that it pays to clean your brush after each coat of paint, if you've emptied the cup. If there's still a good amount of paint in the cup, you can let it sit while you dry the fresh coat of paint, but don't let it sit for more than a minute. If my paint is taking longer to dry for some reason, I'll pick up the air brush and shoot a little more paint through it to keep the tip wet.
But backflushing with lots of clean water between coats is really the key for me to keep my brush working.