Personally, I don't see any real exercise to improve balance. If you really wanted to, maybe take one of those Tai Chi or Pilates or Yoga classes or something. I don't know much about them, but I've heard theyre supposed to improve balance and whatnot.

A better idea (especially for mixed martial arts) might be to sign up with a Judo school. Judo would be an excellent complement to your boxing skills in mixed martial arts (I personally think Judo + Boxing crosstraining is going to be the next big thing in MMA). Judo specializes in throws and takedowns using your opponents balance against them. Through Judo, you will learn not only how to disrupt your opponents balance to deliever knockout power throws, you'll learn to defend the throws and takedowns yourself, as well as learn to have a better eye on your own balance. Judo will also teach you some submissions to use, too, should the fight go to the ground. And in learning to perform submissions, you'll learn to defend them as well, making you a MUCH more complete fighter than boxing alone.

And the best news? Judo schools are generally plentiful and generally cheap. Just make sure that the gym (school, whatever it's called) knows you want to learn with the intents of fighting. Research the school well and make sure there is plenty of full contact sparring. Otherwise, you really won't be benefiting.