In my experience, it's just something that happens. I used to be horrible about falling off at the slightest thing. Now, after riding almost daily for 5 weeks on a horse I trust and in a saddle I am completely comfortable in, I haven't fallen off once. I can sit shies, crow hops, and can canter without shifting at all.
I've helped myself a lot by walking around with my feet out of the stirrups, and I suggest you give it a try. Keep the stirrups on the saddle though, and practice getting your feet into them without looking as fast as possible. That way, if you feel your horse tense, you can get your feet back in them ASAP to minimize the risk of falling off.
You will find that your balance and seat will naturally improve the more often you ride, and the more you get to know the way horses move.