If there's any chance you have the original carton it came in from the manufacturer, that's always good. Otherwise, cram it down as small as possible - CA will steam out any wrinkles. If it has boning, fold it in such a way that the ribs don't get bent, and if it has beading or sequins, put a layer of tissue between them and any other part of the dress they might touch.
This tutorial I wrote is on packing your gown to preserve it, but it might help.