Blaster - The only thing that I can think of, is what brand of thinner are you using? I spray my Tamiya very thin, sometimes as much as 60/70% thinner, I use Tamiya X-20A for thinning. Have never felt the need for retarders or other surfactants, and rarely have build up on the tip. I do keep a jar of lacquer thinner and some Q-tips handy, for a quick swipe of the needle once in a while, just to be cautious.
Mostly Badger and Iwata airbrushes, I generally average about 14-18psi. Hopefully Don Wheeler will drop by to give his thoughts.