I actually blip the throttle with my heel while pressing down the brake pedal with ball of my foot.

my feet look like this when I'm heel-toe shifting:

left foot ---> | \ <--- right foot

I sort of 'kick' the gas pedal while firmly pressing down the brake pedal with the ball of my foot.

works for me, still a need a bit more practice, but when I get it just right it's buttery smooth