I found some $2 t-shirts and iron-on printer paper at Michaels, and created an assembly station on my ironing board. Disclaimer: If you are looking for high-quality t-shirts this is not the project for you, but if you are looking for a novelty (and probably one-time-wear) t-shirt I definitely recommend this approach. My printer had a "t-shirt transfer" setting under properties which automatically printed a mirror image that would iron on properly.
All you have to do is place the printout face down on the t-shirt, iron (I found that 2 or 3 passes with the iron for 10-15 seconds worked best, with a little extra focus on the corners and edges), then peel off the backing (an exacto knife helps here). Here is my final project:
And of course, the project wouldn't be complete without some cute packaging, so I rolled them up (with the STTT logo prominently displayed) and tied it with some cute ribbon.
Who wouldn't want to run in this?