To all of those who thing riding fixed will improve your pedal stroke, I have to disagree. A fixed drivetrain actually hides many imperfections in the pedal stroke because the momentum of the wheel will keep the cranks going, whether you're putting power down or not. And when you start to really do work, it gets hard to tell when you're moving the cranks or when the wheel is doing it. So what seems like a smooth pedal stroke on a fixed drivetrain may not be. The only thing riding fixed is good for regarding spin technique is that it will force you to really move your legs once the speed leaves your comfort zone, and you'll know when you start bouncing all over the saddle trying to maintain 35mph @ 75GI.

I personally think that the best way to improve your stroke is to just gear down and spin. Keep your butt still. The rest will come over time.