A good start is to sit on a Swiss ball and move around on it much of the day.
This will build up your core muscles, the fine muscles in your hips and, most importantly, loosen things up.
Explosive power also comes from removing opposition. Seriously, you probably have many times the power you think you have but can't apply it.
One of our advanced exercises is to just stand for many minutes and consciously relax the pelvis and lower back. If you find this difficult, start practicing the relaxation lying down on a soft surface or, even better, in a pool. It is very important, however, to learn to relax this area whilst standing.
It is vital when training for explosive power to only hit a resistance - a pad or bag. Kicking or punching into mid-air will undo all the reprogramming you have done to remove the opposition.
Next work on being able to relax your hips through all the angles of the kick - stand with your leg out on a pile of cushions.
For punching, imagine you are doing Tai Chi and try to soften your legs as much as possible. Slow it right down and note points where you can observe tension.
Sit in deeper stances and do your rotation in a relaxed manner and you will find all the strength development you need, but it won't work if you haven't trained your nervous system to relax.