For all the men out there who need to know this:
Opening a girl's bra is nowhere near as tough as it looks. We've been doing it for ages, and we usually do it with one hand behind our backs.
The trick is (while facing the girl) to reach behind her back, insert your hand under the strap, fold the hooking part just where the hook holds on to the eye, and the hook snaps open. As simple as that.
Trust me, we girls snap other girls' bras open for the sheer fun of it. And also cos it's such a simple thing to do, and can be done in public, and it will invariably annoy the person who's had it done and cause her a lot of discomfort since she will hardly if ever have the presence of mind to snap it right back in place in public. When you get a bit more confident, you can even do it without reaching under the clothes.
Oh and I'm writing this after going through this.