Think of touching your belt-buckle to the pommel.
Or, if you prefer, you can think of it as 'kissing' your horses neck. That's the classic hunt-eq way to say it. You want to snap over the jumps, and in order to do that you really need your horse to help you lift out of the saddle.
You want to think about touching your bottom to the saddle a stride before the jump and letting the horse lift you out of the saddle so you can snap over it. It's not easy at first, I promise, and you'll probably get left behind a lot when you first try it. But it gets easier :)
Doing this will also help bring your legs a little further forward, and help you shove your heels into the ground. My instructor used to tell me that she should have to clean the bottom of my shoes before I go into the show ring because my heels should be down so far. A good way to work on your heels is to stand on the edge of a stair/step and hold onto the railing while crouched into jumping position. Your heels should be dangling off the end of the step. It hurts your thighs at first, but you'll see dramatic improvement in your heels.