I'm guessing your bindings are too big for your boots. What size are they, boots and bindings both?
You also may have your highbacks adjusted too straight up and of course if that's the case, your boots will tough only at the heel cup. Turn the flad until the highbacks are slightly pushing the boot away from the heelcup. When you strap in, do the toe first so your boot is pushed far back against the highbacks. This will hold the boots as securely as possible in the bindings.
Finally, use this:
SHOE GOO CLEAR
Shoegoo. Walmart sells it, pretty sure more of the big box lowes and Home Depot do as well. Silicone based, dries hard yet flexible. Just smear some on the worn parts and replace as it gets worn off again.