A long time ago in a galaxy far, far away, I'm used to make many illustrations for printing on T-shirts, buttons and so on. Now I'm trying my hand on it again, doing some pictures to submit to Threadless and other online contests of same type. Obviously, I went out of this business for a long, long time and people's tastes have critically changed during this hiatus.
Once upon a time, I made illustrations using traditional media (paper, ink, beer, etc.). These days such things seems to be a Flintstone-ish tech that were been substituted by digital media (minus the beer, it cannot be substituted), mainly vector works. Vectors can be highly scalable and are the format of choice when it comes to screen-printing because they could be printed at any size without flaws (no pixelation or blurring).
I have played once or two times with vectorial drawing apps but I have never enjoyed those experiments.  Now I'm practicing more and more with those apps. Hope I could improve my skills better.