Stretching is very important but I will only touch on stretching the hips. All of your power in your kicks comes from your hips, so you might as well have them nice and stretched to give maximum power.
I recommend this stretch: While in the standing position spread your legs apart about four to five feet apart. Put your hands on the top of your butt and push your hips out forward. You should feel your hips stretching out. You can also rotate your hips slowly and feel the hips stretch differently in differently positions as you rotate your hips.