OK, I'd say there are two main areas that will improve your kicks:

Flexibility
You must work on your flexibility a little bit each day (even if it's a non-training day). You may say "what's the point of being able to kick 2 foot above head high, no-one's that tall" and the reason is simple, if you can do that it means your muscles aren't at maximum stretch when kicking at head height, therefore there's more power available at that point.

One of the best methods of improving flexibility is Isometric stretching. Basically, when you're in a given stretching position, try to pull your feet back together against the resistance of the floor. This basically uses the exact muscles your stretching and after being contracted they relax more than they did before. A rather awful looking website, but the book on stretching is very useful (after using it for a few months I was able to do the front splits, on chairs and the floor and pull myself back up without using my hands) - Stretching Scientifically

Body Movement
When doing a particular kick the body must move in to a certain position. The key is though to minimize any additional movement. OK, let me give an example: to do a head high round house kick (turning kick in Taekwondo) the body has to turn through 180 degrees and lean slightly back.

That's it! So many people then lean off to the side as well or crunch up their body which ruins the kick. Literally, practice (without kicking) just turn your torso from your sparring stance then lean it back. That's all your body needs to do, anything extra is wasteful and will throw the balance during the kick.

I'm not going to go through all the kicks and explain what your body should do, but watch your seniors kicking. You'll find their body moves to exactly the right position, quickly and effectively with no wasted "extra movements/positions". Once you're aware of this idea you may start noticing that you do it when you kcik, that's the first step to improvement.