Cinderblocks. I went to Home Depot, spent $5, and jacked my desk up 18". I figured I'd take a flier on the standing desk idea, but it's working out so well that I don't think I'll bother with anything fancier.
Some kind of an anti-fatigue mat is kind of a must if you're standing on a hard surface. You don't have to go big on that necessarily, either - mine came from the home goods aisle at Wal-Mart.