Stop pinching with your knees and adjust your irons so that they lay about a half inch above your ankle bone when you leg is out of the stirrup
Shorter stirrups should do the trick.
Honestly I would keep your stirrups shorter and really get your heels down. Make sure you are even in both stirrups too. I'd recommend to keep the balls of your feet in the stirrups and rather then having then too far in loosing them is actually better if doing hunter. (believe it or not) If you do loose your stirrup during the show, try and keep your cool (This has happened to me and its hard! But the judges like to see you act like nothing has happened its more impressive) Try to get it back if you can. If it goes too far in try and wiggle it out in between jumps.
I have that problem with a super bouncy horse a ride! My foot ether falls out of the stirrup or slides in and it looks awful! I have learned when I ride that horse to make my stirrups a bit shorter and really shove my heel down and try to squeeze to keep my leg in place. You can ether wrap your legs around the horse and grip or turn out your toe a little to try and keep your foot in the stirrup that way. Hope this helps!