This is a great way to show off your nerdiness or just have a cool shirt. All you need are a bunch of circuit boards (Actually, just get something electronic, smash it open and you can use pretty much anything in there), hot glue, and an old t shirt.
First, take whatever electronic device you have smashed and just poke around in it taking whatever looks cool. ANYTHING will do. Get a good amount of stuff though. Next lay your shirt out and place the objects where you think they look the best, get them the way you like it. Now just heat up the hot glue gun and glue everything down. Thats it. Your done.