unhook a bra one-handed

EnderKR’s way really works.

It’s easy to do your own. Someone else’s is the problem. ;)

...but I suppose it doesn’t really count.

..is to snap your fingers.
Reach around her and gently pull the strap away from her back with your index finger, just enough to give your other fingers leverage. Then put your thumb and middle finger on either side of the strap, push them together, and snap your fingers. The bra comes undone easy as pie, and you usually get a look of amazement from the girl. Totally worth it.

all i can say is practice, practice, practice. and it’s SO worth doing!