Get a colored t-shirt. Black/blue seem to work the best, red also works (but comes out pink... just a word of warning).
Find your favorite stencil art. I used the Aquilla. Print it out in the center of the paper. Cut out the image w/ an exacto knife. You can either use a negative (the space around the item you want) or the item itself...depends on the look you're going for.
Place the stencil on the shirt, and tape it down flat. It helps to put carboard under the shirt too to keep it flat/prevent bleed.
Put a mixture of bleach 20% and water into a spray bottle. If you want a darker mixture, add some coffee grounds.
Spray the area on your stencil. Bleach will quickly change the color of the area it hits. For best results, spray lightly and feather the edges (unless you just want hard edge). Once you're happy, dunk the shirt into a sink filled with cold water (ice cubes help). This will null the bleed effect and lock the color. Let it dry, then wash it once like a regular shirt.
Voila. Custom shirt without much hassle. Lots of really cool effects to be done with the spray bottle as well.
Subreddit for just shirts:
http://www.reddit.com/r/bleachshirts