Like SaltyDog, I can't really help you with the Aztek itself. I don't own one and have no idea what the capabilities of it or what tips you have available.
The key for me, and I'm sure no expert by any stretch of the imagination, is geting very, very close to the surface. As in about an inch off it. To be able to do that I use very low presure, like about 5 psi or so (I can't tell for sure because the gauge barely registers) and very thin paint.
Draw the outline very lightly in pencil about 1/8" in from where you actually want the lines to be. Paint just outside the line and angle your airbrush so the overspray will go into the area you are painting.
Fill in the area, and switch back to the base color. Go back on the other side of the lines and tighten the demarcation up some. If necessary, go back and forth until you get the edge that you want.