How can I improve my pedal stroke?
Pushing down on the pedals is a natural thing. So to improve the "roundness" of your pedal stroke, completely ignore the down-stroke, instead focus on the following:


Continuing the pedal stroke "across the bottom". This will probably feel something like trying to scrape mud off the bottom of your shoe.
On the up-stroke, focus on driving your knee upwards toward the handlebar.
At the top of the up-stroke, focus on sliding your foot over and down into the down-stroke.


Focusing on the non-instinctive 3/4 of the pedal stroke has really helped me improve form.  Using a fixed gear (or staying in a low-ish gear), find someplace where you can spin out your cadence (a slight down-hill but not too steep).  

Focusing on the 3/4 of the stroke that isn't pushing straight down, pedal as fast as you can until you find yourself starting to bounce. Keep doing this, and as you get better at the non-down-stroke, you'll find that the pedaling cadence at which you start bouncing will get higher and higher, as your pedal stroke gets more round.