I can also get away with wearing girls' sizes because I have no hips:
I have discovered girls size tanks at Wal-Mart and Target make great workout and running tanks for me. I can wear the girls size Large and they fit me perfectly. They are even the right length. Also, the price is right. I bought a pretty pink ribbed tank yesterday at Wal-Mart for $3! I can wear the pants too, but they are always WAY too short!