one of the things I try to do is on flat days ride with shorter stirrups so my heels can feel what it's like to be pressed down and stay in position , then on jump days have my stirrups longer and practice keeping my heels feeling like they do with short stirrups
Try this: Stand on your stairs with the ball of your foot on the step. Stretch downward. It stretches out your calf muscles so it's easier to step down into your heels in your stirrup. Also riding with your stirrups 1 or 2 holes shorter makes it easier to practice stretching down into your heel.