A couple tips:

*When you're first starting out, it helps to practice while talking to a friend (preferably a friend who tolerates your eccentricities and/or is a fixed gear rider also). You'll be able to hold it longer while looking at your friend (instead of the wheel) and thinking about the conversation (instead of thinking about falling down).

*Practice trackstanding after making a stop, rather than just jumping on the bike and trackstanding right away. Try this: once you come to a complete stop, turn your wheel in the proper direction, and...don't do anything with your pedals yet! You'll probably find that it takes you at least a full second before you really start to fall over. This is a good thing to teach your body. Beginning trackstanders often panic and overcompensate with the back-and-forth on the pedals, which causes wobble, which causes them to lose the trackstand. When you learn how little you really need to do to stay upright, you'll get much better at staying upright.