Here is a stress analysis of a strapless evening gown.   I just KNEW there had to be a scientific approach to this mysterious ability for nothing to hold up something.  Here is the focus:

Effective as the strapless evening gown is in attracting attention, it presents tremendous engineering problems to the structural engineer. He is faced with the problem of designing a dress which appears as if it will fall at any moment and yet actually stays up with some small factor of safety. Some of the problems faced by the engineer readily appear from the following structural analysis of strapless evening gowns.