When you're riding, stand up strsight in your stirrups for five strides, then fold into the jumping position for five strides, and keep doing this til your legs ache like crazy! Do this in all three paces and you'll soon find your balance and get a solid lower leg. Do you keep your heels down the rest of the time? You could try jumping in the light seat, which is like the jumping position but you sit more upright and your bum just brushes the saddle. This way all you have to do is fold a little more over bigger jumps, and don't have to worry about going from sitting to a jump position. Do some general schooling without your stirrups, concentrating on keeping your heels down. You have to build up 'muscle memory', amend yourself each time your heels creep up, and your body will gradually learn to keep your heels down as it's automatic position, rather than heels up, as it is now.

A stretch that I always do is crossing my feet and then bending down to stretch. Then, cross the opposite legs over one another and stretch down. Also, I do this stretch too: Step onto the balls of your feet on a bottom step of stairs and put all your weight down into your heels and do this for about 10-15 minutes. After 2-3 minutes, lift them up and down for about 10 times, and then stretch again. Hope this helps!