As with your other post.
Keep the knee high and close to the body. This should encourage easier movement and balance. To assist this I keep my ankle cocked in the position I want it to arrive in.
So for side kick, the bottom of the foot is curled to face the side of the knee, big toe up, other toes curled round to form the "sword". For the back kick, keep the the foot up, so the bottom of the heel faces directly down.
Try leaning into the kick (side only) and keep your arms close to your body.
Re-chamber your kick before moving, this will aid balance and movement.
Power is generated from the hips.
Additional forward movement will assist this, but also don't forget to pivot a little on the supporting leg to get increased distance and power.