Often people think that their balance is fine on horseback, but when their saddle is loose, they slip. Here's a great way to improve balance

[edit] Steps

Start out riding bareback with a bareback pad as the bareback pad helps the unbalanced rider feel more secure.
Eventually, after riding with the bareback pad for at least a week or two, move to riding the horse without the bareback pad, and try posting the trot as this builds up calf and thigh muscles.
When you feel ready, get back into the saddle, if the saddle is loose and still slipping, tighten it, and work bareback again, then move back to the saddle. However, DO NOT ride in a saddle that you know is too loose. This is a way to get yourself and your horse injured. Please tighten the girth before you get on the horse. Riding with a loose saddle to check your balance is NOT a good idea.
Ride in the saddle but remove the stirrups to better your balance with a saddle.

[edit] Tips

Riding bareback also really helps english riders and jumpers to strengthen their thigh and calf muscles and keep their legs in the correct position. Also for the jumpers, the stronger grip with the thighs and lower legs rather than the knees keeps better balance over the jumps.
Improving your balance through excercises, such as ballet or yoga on the ground can greatly improve your balance in the saddle.
Make sure you keep your heels pointed down at all times. It increases your centrifical balance.
5 points of balance: 1)heels down, toes up; 2)chest up (shoulders rolled back); 3)legs on tight; 4)hands together (but not too much- you generally want them to line up with the inside of your shoulder); 5)eyes foward and looking straight ahead (not at the ground a few yards in front of you- keep your chin up).

If you do not think you can ride bareback then do not do it. Try being led by someone first before attempting it on your own.
Don't ride bareback on a horse that is fresh or under exercised, though you can, it will be quite dangerous unless you know what you are doing.
If the horse is not yours, or you feel you do not know the horse you are riding well enough, you may want to try riding without stirrups first.
Be extra careful when riding bareback, as it is more dangerous should the horse spook and bolt.
If your horse becomes to much to handle, get off and try lunge your horse to shake off any extra energy.