Tom House has a drill called the "mirror drill" that works on the separation and late shoulder rotation aspects of the pitching delivery. It's kind of hard to describe in words but I'll try...
Start with the feet spread fairly far apart with the front foot at the angle in which it would plant after your stride. Bend the knees and cross the arms. Slide the hips back and forth a few times keeping the head level and then come to a stop in the forward position. The front knee should be flexed as it would when you plant after your stride. The knee should not be forward of the toes. In this position, release the back foot and rotate the hips as far forward as possible while keeping the shoulder in the closed position. Hold that position for a few seconds and the rotate the shoulders. As the shoulders square up to what would be your target, move the spine as far forward as you can while keeping it upright and then hold that position for a few seconds. Relax and repeat.
You should feel a stretch across the torso at the point where the hips have rotated and the shoulders are still closed. Then you should feel an isometric load on the lower back after the shoulders have rotated and the upright spine has tracked forward. Do this in front of a mirror to ensure good form.
Once you have this down, then you can add in the arms to practice opposite and equal arms into foot strike as well as the swivel and stabilize of the glove over the front foot. Simulate the throwing motion and pay attention to how far out in front your release point would be.
You can get a video that demonstrates this drill at http://www.nationalpitching.net