TheAnimal

First and foremost, practice your knee strikes. On a bag, on a pad, whatever- thats what gets them powerful. After that, you need a strong overall body period, specifically a strong core. Any and all ab exercises you benifit from, deadlifts, squats, etc will all help. Another idea is to get those ankle wrap things and attach it to a cable machine, and put on a small weight and actually move as if you were kneeing something. Its the same motion, only with a bit of resistance, which is a great way to train things.

train each body part once a week at most. eat more calories. and lift legs!!!

You know, the number one way to make your kicks powerful is to practice kicking. So never sacrifice that. After that, do some squats and legpresses and all that to build up the muscles.