I've fallen off tons. I've always heard that if you haven't fallen off, you aren't riding hard enough.
I've never been thrown when a horse acts up. I usually get angry, and for some reason, anger turns my butt into super glue. =] I do things like forget to turn. =P
I used to take vaulting lessons, so we jumped on and off walking, trotting, and cantering horses. It was intense, but it did incredible things for my balance and secured my seat a LOT. I mean, you try flipping yourself in one move to face backwards while your horse is trotting or cantering, and then flipping back to the front. It really helped me learn to move with the horse, and I think that's why I can stay on a horse acting up, because I've learned to really feel and react and move accordingly. I'd recommend vaulting to anyone. =]
BUT, if it makes you feel any better, I've never fallen off of my mare that I've been riding for two and a half years. Actually, no one has EVER fallen of my mare in her 17 years of life, until my sister rode her a few months ago and took a plunge. But that was the first time, lol.