Airbrushing T-Shirts, Jackets and etc. is just a whole lot of fun and a great way to make money. You don't even have to be good. It was amazing how bad some of the stuff was I did when I started that people absolutely loved. If your lucky enough to get into a big mall or extreme high traffic area you'll work at a rate of about $5.00 a minute before taxes for as long as you can stay awake. Not to mention how many garments you or the store you're in will sell for you to paint on.
In order to be highly productive and create that kind of cashflow, you'll need a big silent compressor, capable of pumping 55-80psi all day long, multiple airbrushes, stencil materials for complex repeat designs - a toothbrush and Q-tips and the ability to paint extremely fast, do the math and talk and "put on a show" for the 50 people standing right behind you.
On a side note, if someone walks up behind you and mutters, "Oh that looks easy, I can do that", it's probably another pro just messing with you! If not, let them try, that little tiny air hole in the top of a paint bottle that nobody sees lets paint drain out very fast when turned the wrong way! For the best light fast permanent paint available see: Aqua Flow, as used here.
Also, Airbrush is a fast tool for painting on T-shirts, jackets and etc. Most of the artwork on this page was created in between :10 minutes and 1:45 a piece.