As an interesting alternative to rope or cloth bondage, try tape. Doesn't have to be the classic duct tape, wide brown plastic packing tape works fine and is even more cruel because it crumples into spiky folds.

Wrap tape round the base of the breast just as you would in rope bondage (but more easily because there's no tendency for the breast to slide out of it even if it's soft). Keep on wrapping as far forward as you care to go, till the blown-up remainder is sitting on a pedestal of tape-squished flesh. Add a strip or two of tape on top, from the nipple up to the collarbone, to support the weight. You will be amazed how far you can get even medium-sized boobs to stick out this way.

Then have your sub put on a teeshirt and take her out to a bar or a mall in her Bondage Wonderbra, and watch the guys drooling. You would never have guessed you could create so much pain and humiliation with a few yards of tape.

Taking it off later is a whole nother treat.