I used to go to kickboxing classes where we had to spend 10 minutes stretching first, but in the last 2 years I think we save the stretching for after the workouts. We were never given the reason why. Just recently at my KM class however the instructor did explain that the body gains more flexibility by stretching after a workout than right before.
Before you work out you should warm up. A good indication will be you are breathing harder than at rest, but not gasping for breath.
You also need to loosen all your joints, especially the ones you are about to work out with.
You need to stretch your muscles, tendons and ligaments through stretching exercises that resemble the the motions of your main work out.
That is front stretch kick, side stretch kick, inside/outside kicks, outside/inside kicks and rear stretch kick. Swing your arms together then back, one up and one down alternating.
These stretches are characterized by a full range of motion starting slow then moving to your full range. They should be controlled and you should not bounce.
Static stretches are best after your workout as you are warming down. A muscle cooling down will get the most benifit from your stretching program. You can add resistence to these streches, by stretching as far as is slightly uncomfortable, have someone hold the stretch for you and you try to returne against the stretch, hold for a few seconds the relax, then immediatly go back in the stretch, you will problably be able to go a little further.
This works for improving your jump height or distance, squat and jump, squat, tense, jump, and squat, jump, you height should be higher on this last jump.
There are several good books out on stretching.
