1) get a ponytail elastic
2) loop through button hole (you know the way bookmark tassles are looped through the holes punched in bookmarks? like that.)
Here's an image of what I'm taking about via the mommylist blog. Basically you're just creating the external loop closure you see at the neckline on a lot of vintage dresses. It's possible everyone knows this trick by now, but on the off chance you didn't, there you have it.
Edit: I wanted to mention that if you've got pants where the zipper slides down, it's another opportunity to put your hair elastics to work. loop through the zipper end and then around the button to keep your zipper securely done up. This works best with smaller elastics.
x-posted to fatshionista