Take your old shoes with you (or your "day" shoes if you a) haven't been running long enough to have worn your current ones in or b) your current running shoes just smell TOO bad). This will give them an idea of where the shoes are worn down, which tells a lot about your gait.
Also, take your running socks with you so that you will know if the shoes fit properly with the socks you will be wearing with them.
Oh, and wash your feet!