These:
http://www.jumpforjoy.org/products.html
They make keeping my heel down so much easier. I could really feel the stretch. I could never feel that before no matter how much I forced them down. But what really surprised me was how much my jumping improved. Before I couldn't jump tiny little cross rails to save my life. I had no balance, I always fell onto my horses neck, I lost my stirrups constantly and I couldn't jump without holding onto the mane and putting my weight into my hands. Now I've had a complete turnaround.
I had perfect balance and was with my horse through the whole jump. I didn't have to hold onto the mane or even touch the neck. And my stirrups stayed in place. And talk about a confidence booster! Now I can try jumping in a show without worrying.