My instructor at the track showed me this. Keep your foot parallel with the gas pedal (ie keep your heel straight with the gas pedal, not under the brake).
When you want to brake, just move your toes (top of your foot), not your heel. Always keep your heel on the bottom of the gas pedal when you want to down shift.
Before I went to the track, I've tried to use my toe/side of foot to downshift and I had lots of problems. I either broke too hard or gave too much gas. Sometimes, I also found myself pressing the gas while braking by accident. For me heel/toe works great, but for others it may not.
With the gas pedal in the link below you can't use your heel to rev up because it is too short:
http://a332.g.akamai.net/f/332/936/1...ition4.350.jpg
The gas pedal on Si is long and it touches the floor, thus allowing you to use your heel. If you look at the pictures in earlier posts in this thread, you will see that the bottom of the gas pedal in the Si is much thicker/wider than the top. It has been designed for that reason: heel/toe shifting, NOT toe/side of the foot.
Here is a link explaining it a bit further:
TurnFast : Heel-Toe Downshift