I just took an old shirt (ok I bought it from the little boys section in Macy's... for $4. I am a sucker for brand names, so of course it's Ecko Red... sorry to the environmentalist in me). I cut the sleeves and made the front boyish on purpose. I then cut straight down the middle in the back and made it smaller, sewed the bottom up, cut holes for the laces, and laced it up!
P.S. I used the thick laces from my Phat Farm shoes... (throwing out brand names again).