Paul Hanlon, the talented and insightful lead hairstylist that I met backstage at the Armand Basi fashion show, gave me a great professional tip on how to prevent your ponytail from drooping. He often uses Stretchrite Round Cord Elastic ($1.10), a five-yard piece of elastic that allows you to secure your hair tightly.
Just snip off the pieces as needed, wrap it around your hair a couple of times and then knot it.
Paul claims that this thin elastic cord works better than a regular hair tie because you can create more tension; the result is that your ponytail stays perky. I can attest that all of the models did indeed have sprightly ponytails throughout the show, despite all of the product in their long hair that could have weighed it down.
Stretchrite products are sold in most crafts or sewing stores, and they're inexpensive. Think you'll give it a try?