No stirrup flat work and you focus on keeping your heels as low as possible! You can also do it with your stirrups on, do it the way you focus best on your heels down. Then you have to do it everytime you ride until it becomes a strong good habit! (~7 days)
And everyday stretch your legs 1 min each so that your leg can get used to being strerched.
Personally, I have found not gripping with my thighs has helped. I used to grip so much with my thighs that my heels would come up. I now ride from my lower leg & actively think about keeping my heels down. All my weight is through my heels!