Here is how we used to work on this problem.
1- Spread out on the bottom half of the mound with your stride foot already down. Face home plate with your front hip and front shoulder. Allow your back foot to lift as you throw, but just rotate your hips and shoulders and throw. This is actually exaggerating staying closed.
2- Once you get the feel of step 1… Go to the rubber (stretch position) and stride straight towards the plate without leg lift (slide step). Concentrate on staying closed (exaggerate this). You should be striding closed (front hip towards plate) and rotating hips right into landing… Keep front hip closed and shoulder pointed at target as long as possible (exaggerate). Do not over throw while getting the feel of this!
3- Once you get the feel of step 2… Out of the stretch position use leg lift, but bring lift foot back down before striding towards the plate. Basically doing step 2 after the leg lift. Concentrate on front hip and front shoulder. Exaggerate staying closed. Get the feel of keeping hip and shoulders closed.
4- After step 3… Go to full windup… Come out of leg lift concentrating on staying closed. If you are closed coming out of leg lift, you should be able to stay closed in your stride.
5- All of the exaggeration in above steps should now keep you closed enough so the you’re your hips should rotate right into landing with your front shoulder staying closed just a millisecond longer. Put together this will look very quick but very smooth and it should definitely increase velocity as well as control.
6- Once you get the feel of this… Go to stretch position… see the target… close your eyes… concentrate on your body especially hips, shoulders and arm… and deliver (throw). Sometimes the eyes become our worst enemy, our eyes allow us the ability to adjust to mistakes… The perfect pitching delivery should not have any adjustments, it should repeat the same time after time. With your eyes shut, you will easily feel your delivery, anything else (even a slight change) will feel uncomfortable.
7- Try to find a real good pitching coach/instructor to watch you… video yourself, if you can’t find one.
Sometimes the problem is more related to the shoulder than the hips. Opening the front shoulder too soon is a very common problem in young pitchers. Results include; arm drag, lack of velocity, control problems, lots of effort, and potential serious injury.
Others might disagree, but I believe the hips and mid section are the most important things for most pitchers. At release point your belly button should be pointed exactly at the target! This is only true if your belly button is located in the center of your belly!
There are those who over rotate, but in stop action, in every single case (I think) the belly button will be pointed directly at the target when the ball is released!
I hope this helps a bit. Others might (probably will) have other opinions.
Good luck