Well, stinging nettles if they grow where you are might work. On the other hand, there would be some nasty swelling from memory, and it'd be more than a one day thing. It's along time since I've had a nettle sting, though, so I'm not sure.

But sandpaper would probably work well enough if you've got a fairly tight bra... although you might want to take a spare normal one...if you move around alot in it, there could well be bleeding. So be very careful and use a fine-grade sandpaper to start with. You could strip it out and go a bit rougher further down the road if you feel like it.

Expert opinions?