If movies have taught us anything, it's not to go into a bra unlocking situation with a cocky attitude. It will always end badly. The thing is that women do and undo bras at least twice a day every day. They are REALLY good at it. To think that you would be anywhere near as good at it without any practice is such a man attitude. The thing is, you're going to need to practice... on your mother. Just kidding.
Go to bra store and guy one. Put it around your pillow and practice the one handed technique. You're going to need that other hand to do other things, like texting your friends, "dude! I'm hooking up with a girl! It's awesome!" At the end of the day, if you don't want to risk looking like a fool, just tell the girl, "take that bra off and show me those titties."