The first time I went wedding dress shopping with my friend Kris is when I learned about the importance of the bustle. For the uninitiated, very simply, a bustle is when you take some extra material of the dress and gather it in a variety of ways. It not only helps keep extra fabric out of the way (ie for walking or dancing) but it can aesthetically transform the dress as well.
I thought this illustrated the concept beautifully:
(wedding images from here and here)