I had this problem, as soon as I did something of a certain difficulty, my heel went up. I then took off my stirrups, and did the whole exercise stirrup less, focusing on having a long, solid leg, keeping it as if I wanted my legs to touch the ground. However, I did keep my heels up, whilst elongating my leg, and I help this helped GREATLY when I rode with stirrups again. It also greatly improved my position!