This thread is really funny, only because I have just recently started falling off my horse, accidently, of course. I have fallen three times in the last week! All I can say is that you have to try to isolate the problem. For me, I was freaking out right before the canter, so I would lean forward, and when the horse stopped, I toppled right over the shoulder. All you can do is figure out why you might fall, and try to fix the problem!
Also, immediately get back on a finish on a good note!
Yesterday I was a horse. Today I'm a horse. Tomorrow, I'll probably still be a horse. Sigh! There's so little hope for advancement.