Holy hoopskirt, batman!
Actually, there's a couple of ways you could pull it off pretty easily. The first step might be in making the dress as separates - a bodice, and the skirts. Whether you make the skirt as a true under-skirt and over-skirt and attach them together at the waistband, or make it a fake overskirt (the underskirt is only the front panel), you can probably get away with a snap or even a tie in the back that you undo while people are distracted and then let them drop, so you can step out of them. Getting out of the bodice is a little tougher but here's where layering it right, doing a twirl, and having other people in your group or designated stage ninjas help grab/pull off the top layer.
Either way, you do have options to make it work. It'll take practice to make the transition smooth, but maybe that'll give you some ideas how to construct the princess dress itself to make it easier to get out of.
Founder and Vice-President, Madison Area Costuming Society, a chapter of the International Costumer's Guild
Strange Land Costuming - www.strangelandcostumes.com