Stand in your stirrups and focus on stretching them down, then as you sit down try to keep your legs in the same position, then just carry this feeling with you over and towards the jump... easier said than done :P
Ok! So when I am warming up I do my two point at the walk and trot keeping my heels down as far as they can go! This REALLY stretches them out and makes it easier for them to be far down when it's time to jump! When at the trot I also go around the arena once or twice doing a pattern of up up down, up up down. Up for two beats sitting for one. This helps my heels as well. I really hope this helps you as much as it does for me!!
It's really something you always have to think about, then it'll become natural. Maybe get someone to say if your heels are down over the jump or not so you get the right feeling. Also if you legs are more forward (on the girth rather than behind) then your leg doesn't slide back over the top of the jump and you won't have as much of a problem with your heels coming up. Also freejump stirrups are sloped downwards to encourage a deep heel (I own them but worth a try?)
Try and think about all of your weight in your heels and it'll help
What I do, is I just put all the weight in my heels! It's helped me keep my heels down tremendously