I never leave a dressing room without shimmying a bit in the dress I'm trying on. When you go into that fitting room with your arms full of slinky black silk, do not leave until you have shaken your groove thing for at least a full minute. Why? Any strap, slit, or button that's going to pop, ride up or cling is going to do it now, in the privacy of your dressing room, not on the dance floor surrounded by your friends and the principal.
Also, carry the following items in your purse for 911 situations:
lots of safety pins
bobby pins for hair mishaps
double-sided tape*
a needle and some thread
a small scarf or wrap you can tie around yourself if a seam should burst
*And I'd like to take a minute here to sing the praises of double-sided tape. It's the fashion version of duct tape. It's magic. Do not leave home without it.