As a quick note,
I never use my heel on the thottle pedal. It's more natual for your foot to go outwards, and it's more natural to give the throttle with your ankle power.
So when I heel-toe I keep my heel on the brake, and move my toes to the throttle. Hondas are particularly easy, because the gas pedal's metal arm wraps around the top of the brake. Try this.
With the engine running, and the car stopped, Put your heel on the brake pedal. (Depress the brake of course) . Then put your toes anywhere. Up in the dash, to the side, anywhere in the general direction of the gas pedal. Push your toes down. Chances are, you'll hit some part of the throttle pedal.
-> Steve
Practice Practice. I heel toe during all decelerating shifts. On the race course I do it in a performance orientation, and on regular roads with passengers I do it to maintain smooth shifts that don't jerk people around.