Spooks are different as first, you never know when they are coming and two, they happen much faster and usually in more than one direction at the same time. My last horse, an OTTB, was sort of spooky. He was one where we could go around the ring in the same direction passing the same items several times at the walk, trot and canter and then the sixth or seventh time we took the exact same track, suddenly the boogey horse would pop up and he would spook. Yet, the birds we have in our indoor arenas, the ones I refer to as kamikaze birds, could fly right into his chest and neck, literally, and he wouldn't twitch an ear. Te main problem is that I never knew what would cause him to spook so desensitization was near impossible. I did know, however, that he hated pink and purple flowers. He would walk by yellow, red and orange with no problems but if we went around a jump with pink and purple flowers, he spooked every time. I was always told horses can't tell colors but I swear that horse could.
About the only advice I can give is that with a spooker you have to stay supple and loose. If you tighten and pinch the motion of the spook becomes exaggerated in your body. Unfortunately, the natural instinct is to keep a tight grip in the event of a spook which actually makes it harder to stay on. I don't have problems so much with what I refer to as the butt scoot..where they spook forward..it is the sideways, drop the shoulder and duck sideways that I have a hard time sitting. It also doesn't help if the horse continues to spook past the initial sidestep. My current horse is very good about that..he may take a hard step sideways but he stops at one step..he doesn't continue across the arena. His standard spook, if he does it at all, is just that hard stop with the front legs.
The other thing is to try and figure out the clue the horse is looking at something prior to a spook. In my last horse's case, he tended to tip his head towards the object as we were coming up to it and the ear on that side would follow...sideways to front as we approached. IF (and yes the double caps were deliberate :) ) I managed to catch the clue he was looking hard in preparation, I could do a simple shoulder in or haunches in (depending on the location of the "boogey horse") and that way his attention was not only back on me but my weight was in the correct direction of where he might spook to..in other words, I had more weight and thus more "stickiness" on the side to where he was going to spook if he did so. That way, as he spooked away from the less weighted side, my weight was automatically balanced just by his movement.
This particular horse was very suspicious of many things to the point a darker colored area of dirt in the arena would cause him to stop and literally sidepass in a circle around it snorting the whole time. The first time he saw the sunbeams on the arena floor as the sun was shining through the closed arena doors, he refused to move forward over the beams...it took 20 mins of moving him in between them (using the sunbeams sort of like poles) and then turning him through them in both directions before I could get him to go "over" them in a straight line.
He didn't spook often, at least not hard enough to even make me blink, but when he had a hard spook where I missed the warning sign or it was just something where there WAS no warning (such as two dogs racing into the arena right in front of him), he dumped me every time...on that last one though..I landed on my feet :).