My horses have been through and continue to go through serious desensitization training. Every two weeks at my barn we have a complete course set up with balloons, flags, tarps, hanging things, noisy things, etc. Yet my horses still can and do spook on occasion. For horses it's not just about getting used to things, it's about visual and hearing perception and even their mood. So to me it's not about stopping a horse from spooking, but about being able to stay on "almost" no matter what.
If a horse bolts and you go over the back of the saddle, something is wrong with your seat. Same with spinning, scooting sideways, etc. Our minds can't react in time for our body to adjust consciously to a horse's movement. That's why I believe in using the receptors in our hips, knees and ankles that telegraph movement to our brain before we are conscious of it. We use these every day when we are walking. That's why you can trip and catch yourself before you fall. It's a reflex.
We can tap into this reflex the same way we do when we're walking: by putting a moderate amount of weight down through our leg and onto the balls of our feet in the stirrups. This means your leg has to be balanced underneath your center of gravity. Then, just like it would if you were skiing, your body will automatically adjust for changes in the horse's position: side to side, forward and back.
If you're sitting on your butt on a stool, someone can easily push you over. If you're standing balanced with your feet on the ground over the stool, then someone will have a very hard time pushing you over.