Petals... keep your foot as flat as possible during the entire pedal stroke... at the bottom of your stroke, it's kind of like a sensation of scraping your foot backward. You'll be engaging your hamstrings to pull your leg up the back of the stroke instead of just pushing down on the front of the stroke. At the top of the stroke, some people have suggested thinking of moving your foot straight acroos the top instead of making a round motion at the top. I'm still trying to get the feel of that one, but it does seem to help.

Hope this helps a little!