Since going clipless, I've begun using three techniques to help focus on my pedal stroke:

1) The Lemond knee-to-handlebar/mud-off-foot image. This was especially made clear to me after Floyd Landis' Stage 16/17 rides. His coach compared the two rides on OLN and compared how Floyd's knees snapped up at the top of his pedal strokes all through Stage 17.

2) To reinforce the mud-off-foot idea, I lead with my heel through the bottom of the stroke. This helps a lot to keep my foot level.

3) I picture my feet going around the pedal stroke as the hands going around the face of a clock. This really helps me to keep my stroke circular.

I also periodically stretch each leg out at the bottom of a stroke and pull my heel down hard. It's a good stretch, and just one more reminder to keep my heels down. All of this is, of course, much harder to remember once I get fatigued