Just this month I "got it" with my jumping, in that I consistently go with the horse and don't get shaken loose over the jumps. The difference for me was the difference between an insanely hot day and a normal day. On the hot day, I was having to really press with my calf to get the horse to jump, and I went with him beautifully. On my next ride, I was back to flopping. My trainer explained the difference was the extra leg on the hot day. From that lesson on I realized HOW MUCH lower leg you need on your horse to keep from having a leg pendulum. It's way more than you use at walk, trot, canter or poles.
I still grab huge chunks of mane, and I'm no made jumper, but at least I no longer have floppy everywhere and heels coming up.
I am also a fan of George Morris in Practical Horseman magazine. For every rider having trouble with form over jumps, he tells them to do lots of work without stirrups. It is not just balance and experience but the strength of your legs that carries you safely over the jump. Mr. Morris suggests 10 - 15 mins posting or cantering without stirrups at the beginning of every ride.