Step 1: Pack an extra hair elastic in your purse, or hide one or two among your wrist full of bangles.
Step 2: Before entering the "ladies room", perhaps even while you're waiting in line, gather the fabric of the dress below the knee, and use the hair elastic to secure it in a knot above your knees. Look ma, you've got two hands!
Step 3: Use the restroom. If unfamiliar with the proceedure, ask a friend.
Step 4: Remove the hair elastic and continue living your life.
Thakoon Printed Maxi Dress, $875, available at My Theresa.