If you've never used fashion tape, it's basically double-stick tape made for fabric and skin. Why on earth would you need that? Tape down your strapless gown (to keep it from slipping) and use it to keep bra straps from showing. It'll even fix a ripped hemline in a pinch (so that you don't have to fuss with safety pins).
Hollywood Fashion Tape, $11.75/box, TheKnot.com/shop