Umm, hmm. I haven't really practiced kicking all that much for quite a while, but I guess I can tell you what I did.
A good powerfull roundhouse (just like a good powerfull anything) should come from your "center" - that great tangle of muscles in your thighs, hips, abs, and lower back. So try to be conscious of connecting it all together instead of just flailing your leg. I got a target object, like a chair, so I could practice kicking over it to make myself aware of how high my foot was arching and where it was coming down.
For a rearside roundhouse, I'd pivot towards my back leg while shifting my weight onto the leg closest to the target. This sort of makes a wind up in the hips and prepares you to keep your stability when your kicking leg leaves the ground.
I'd lift the kicking leg and throw it in an arch, heel first, releasing the tension in the hips and turning the hips smoothly along with the leg, pivoting on the post leg. Try to keep the heel down on the post leg for stability. Be conscious of keeping your weight down into the ground through the post leg and extend up through your spine out the top of your head, or else your leg and your upper body will sort of act like floppy, unbalanced counterweights.
For an advancing kick, land the foot in front of you. For a stationary kick, turn the hips a little further and land it behind you, where it started. If the foot lands with a "THUD" then I think there was too much intention in the kicking foot, and the kick was probably weak and off balance. Set your foot down how you want to where you want to. Make sure the kick archs exactly how and where you want it to.
And when you get good at that, practice kicking things like heavy bags that don't allow for follow through, so you can have experience with your energy being stopped and recovering. Styles differ and I'm no expert, so if anyone would like to point out some way that I'm wrong, please do.