"I figured this trick out just prior to my wedding, as my dress was (as you can see) particularly unruly and I didn't really love the idea of employing a small army to help me keep it out of the toilet water (though my bridesmaids were willing to help, bless 'em). Had it been a bit easier to get in and out of, I might have just taken it off, but no — that was a complicated, multi-person operation as well, involving a whole bunch of lacing and carefully placed safety pins and chicken cutlets — not to mention being topless in a public restroom.
And of course, I'm the kind of person who has to pee every thirty minutes, and more often when I'm nervous.
Luckily this little trick is cheap and amazingly effective — plus it's easy to stash some extra trash bags in the bathroom, or find some in most venues if you run out. (A garment bag works well too, and lets you skip Step Three since it already has an extra hole.)
Step 1: Start with a poofy dress.
Step 2: Add a large, sturdy trash bag.
Step 3: Tear a hole in the bottom of the trash bag that is just wide enough to fit your feet through. Aim to keep it small- it will expand as you pull it up your legs, but you want it to fit tightly.
Step 4: Step into the bag and pull your feet through the hole. This is WAY easier if you take your heels off, but in case of super gross bathroom floors, it can be done. Stand near a wall so you don't topple, though.
Step 5: Pull the top of the bag up and start loading your dress-fluff into it, while keeping your feet pretty close together so you don't widen the hole too much. Some of the dress-fluff will inevitably try to escape through the foot hole, but it's not a big deal and you can just shove it back up in there.
Step 6: Shimmy the bag up your body carefully, making sure to get all the dress-parts, ribbons, trains, etc into the bag.
Step 7: Once the foot-hole is around your waist, you can gather the slack at the top of the bag and hold it in one hand, leaving the other free for… bathroom… tasks.