The key to improving anything is to break it down and work on each aspect one piece at a time.  Kicking is exactly the same.

Here's a short list of key kicking elements

1. Correct technique

2. Balance

3. Flexibility

4. Accuracy

5. Power

There are others, but this is a good start.  So, how do you improve these elements?

1. Correct technique is essential.  The basics you can pick up from your teacher or there are plenty of internet resources available.  Always remember though to keep your energy moving forward - avoid leaning away from the target (you send some of your energy away from the target) and always get your hips into the movement

2. Balance is the basis of efficient movement.  Practice: one legged standing, doing techniques slowly with the eyes closed and one legged squats.  These exercises will boost your balance and enhance your whole art.

3. Flexibility is not just for the high kickers.  Even if you don't practice kicks above the waist, increased flexibility allows you to kick faster and harder.  Devote time to dynamic stretches to enhance flexibility 

4. Increased accuracy = increased effectiveness.  If you can kick where you want, when you want, then there's not much more you can want!  To improve this always focus on where you want to hit.  Practice on a heavy bag - Put spots of tape on the bag and attempt to hit them whilst kicking.  Start off slow and build up to full speed.

5. A great deal of power has to do with correct technique.  Follow the guide lines above and ensure that you follow through with your strike.  Hit through the target.  Heavy bag work will improve your kicking power.  Also, check out squats and weight exercises to add more power to your technique. 

There you have it.  Follow this advice and see how you improve.