Here's a video showing how your feet should be positioned while riding.

NOTE:  This video shows GP Shift pattern, meaning the rider presses down on the shifter for a higher gear.

Some key things to notice are:

- The resting position for the foot is on the "ball" area.  The toes are not hanging low.

- The rider moves the foot forward, does the appropriate shift... then replaces it back to the resting position.

- By always being on the balls of his feet, the rider is able to effectively move his body around on the bike.  It also keeps the toes from touching the ground.