A professional driving school would do wonders. In bondurant, you spend 2 hours on the first day driving vettes around an oval just heel-toeing.
They set you up in a big parking lot with a cone at each end. it is set so that you exit the turn in second, shift into third at the middle of the straight, and then hell-toe into second for the next turn. You wind up puling a shift every 5 seconds or so. instructors jump into your car to make sure you are doing it right.
They take it pretty seriously. But....on day 3 or 4, you do it second nature.
I heel-toe 80% of my shifts on the street that involve second and third gear braking. And on the track, 100%
Think of how hard it is to find the time to work at heel-toe. It would take months to get that much practice.... call bondurant!