Did you see these ponytail elastics at anthropologie? They look exactly like a knotted piece of fold over elastic to me. To make your own take five or six inches of fold over elastic (I purchased mine from an etsy supply shop- it comes in a bunch of colors), fold in half, knot the ends together, put fray check on the raw edges, allow to dry and trim off any excess fuzzies- all done.
9/26/11- I LOVE these hair elastics- they don't leave your hair all bumpy and rippled like the generic hairbands from the store. I'll be making these again.