Tinyliny gave you some great suggestions. :)
Another exercise I like to do is to stand in my stirrups, legs under me, not holding on to anything and just hang out there. I find that it really helps me understand being over my horse's center of balance and how to stay in my horse's center of balance, which makes me have better balance as a result!
Another good one is riding bareback! I've found that trotting bareback, it seems most effective without a bareback pad but with a pad works too, really has helped me be able to sit my mare's big, very animated, trot much more effectively in a saddle. I think it's because when you're bareback, it's hard not to follow the horse's motion so you kind of learn exactly how your horse moves at the trot, instead of just what your body thinks the horse should be doing while you're trotting. Also with bareback, at least for me, I can feel myself stiffening up at the trot much better than I can when I'm riding in the saddle. And since stiffness is basically the arch-nemesis of trotting, I can figure out what makes me stiff and solve it much more easily when I can actually feel it.
Fabio - 13 year old Arabian/Lipizzan gelding
Rest peacefully, Lacey.