Oh, and while we're at it, here's the best method I have found so far for strapless bras, bodices and the like. This one is not technically strapless, but the straps don't do too much to keep the cups in place, so I think it counts. :-)

There is a thin strip of "underwire" between the cups and the main fabric which is constrained to the body. As long as this is located in the crease under the breasts, and as mentioned above, the fold resistance is high enough, the distortion due to the constraint is minimal, and if the cups are modeled to be tight to the breasts, it is enough to hold them up. Although it seems obvious in retrospect, it took me a long time to come up with this method, so maybe it will help someone making or converting similar clothes.

-- I'm not mad at you, just Westphalian.