in addition to pulling your heel out, push your toe in. it helps me to think of squishing a bug (or cigarette). you shouldn't need to take any spills, just practice in the parking lot for a few minutes before you hit the traffic.

Phiber: the clipless pedals really help give an even pedal stroke by letting you pull up as well as push down on the pedals. give 'em a try- your LBS should let you try out the pedals on a stationary bike.