Some techniques to improve balance include getting into a horse or front bow stance, slowly go through all of your kicks 10x, bringing the leg up to chamber, turning between 90-180 degrees slowly, extending your kick, turning back to the original position, then down to the original stance. Once you can do each kick up to 10x without losing your balance, you'll be set.
Another technique is to practice your kicks outside in a yard with uneven grass, such as small mounds. If you master the above exercise, then go to an area with uneven grass, you'll find that you'll once again be losing your balance, though not as bad as if you didn't master the above exercise.
Another way to make it even harder is to close your eyes while practicing the slow kicks because the eyes are a huge factor in your balance. once you close them, you'll increase the connection between the brain, inner ear and muscles and further improve balance even more.
Adding a light weight of 1-2 lbs to the kicking leg, but none to the standing leg, then practicing slow kicks will further increase your balance as it forces your standing leg and brain to adapt to uneven weight distribution. So once you can do that well with a weight on the kicking leg, you should have much more solid footing and thus, solid kicking without the weights.
Also, remember to keep your foot flat as many arts make the big mistake of going up on the ball of the foot when kicking, especially the karate arts. This is very poor because you're losing all your rooting to the ground, which is where balance, stability and power come from.