Keeping your knee up whilst kicking
In Taekwondo wherever you point your knee is where your kick will end up. Therefore the higher you lift your knee the higher you will kick. A common mistake is that students drop their knee after they have kicked; this has a couple of effects on your technique. Firstly the sudden drop of the knee causes you to lose your balance; plus it makes additional kicks much harder to perform.
A kick has three points to hit, the same as every Taekwondo technique. 1; the preparation, 2; the technique and 3; the recovery.
So before you carry out a kick, life your knee, execute and then make sure that the knee is returned to exactly the same position as it was when you started.
EXERCISE: Lift your knee up the front of the body and try executing multiple kicks without dropping your knee and putting your foot on the floor. Everytime you recover you should come back to the same initial position.