One thing that contributes to good airbrush finishes is to consider the airbrush as a kind of brush, not the same as a spraygun. An airbrush is not a tool to keep far from a surface and cover a large area quickly. The spray gun is more of a device to get the same kind of coverage you get with a brush, but without making brush marks.
BTW, there is a whole forum for airbrushing and painting threads and brands, models of airbrushes, as well as useful tips, are a very frequent thread there.