Well, if you're right handed take the ball in the right hand. As the opponent comes up, give him a wild step to the left and also (this is crucual) move your head to the left. If he's fooled by that then immediately blow past him on the right by crossing your ball to your right and going in that direction as fast as you can.
Little tip - don't do that very often, or they'll start expecting that from you and won't bite the crossover. A solution to that would be to learn different drills.
I have page where I talk about the crossover. It's at http://www.best-basketball-tips.com/...crossover.html
You can also go to youtube and search some clips from Allen doing the crossover. No one does it better, for sure.