Off of horseback.. anything with an exercise ball. Even sitting on it helps with balance so much! Yoga is great and any kind of cardio workout will help you remember to breathe through things while you're on a horse. Stretching before and after riding is very beneficial too. Also, dancing! Nothing like playing a few Wii dance games to get you loose and your blood circulating.
The most important thing to work on is abs though. A strong core means for a more balanced ride.
As for on the horse.. walking without stirrups or better yet bareback, will help improve your balance. Also riding with no legs while on the lunge line (leg hanging down, bend at the knee bring it back towards the horse's tail. reach with your hand and hold the toe or entire foot) on both sides, making sure to sit on your seat bones and push those knees so they're almost pointing down. Also another one on the lunge line is to bring your knees to meet at the top of your saddle (a little trickier for western saddles) and put your hands either on top of your lap or on your hips.
Without being on the lungeline.. working on 2-point. It'll help with your balance a lot because it gets your legs where they need to be. Also doing hip figure eights at the walk (left hip forward, circle to the outside with it, bring it back to the middle, right hip forward, circle to the outside with it, bring it back to the middle.)
Finally.. working on that sitting trot. Start with a posting trot if you aren't feeling comfortable sitting yet or if your horse wants to go go go. Get them to slow down sit the trot and aim to keep your horse on the rail.. then do circles.. figure eights.. weaving, serpentine. All at the sitting trot. You can also do it in 2-point regardless if you ride western or English.
Good luck :)