I'm super lucky my horse is such a natural jumper, I'm mostly just along for the ride. It is very important to be able to see distances and ride to them, and easier to learn if you're on a natural or very schooled jumper. I count A LOT, even just riding around. It is very important for any discipline you do. In dressage one has to count to flying changes, count to the end of the ring to know when to come back from a lengthening and just generally for making an accurate test. In jumping, it's obviously to be able to get over the fences more easily. The most important thing though is to keep the rhythm. If the horse gets out of rhythm, it is nearly impossible to see distances for us normal people. Keep a big, rolling, 1 2 1 2 1 2 in the canter and only slightly adjust at a time, sit up more to come back and give more room in the contact for more forward. Never make a huge change before a fence.
I've actually improved this since I wrote this thread. Most of my problems stem from my horse slowing to the jump. (This is her issue, not mine.) If she slows too much, I go really deep. If I push too much, I take the jump huge. Lately I've been able to give just enough leg that the striding comes out perfect.