I have my players do something similar to what Chris suggested as part of our warm-ups at every practice. Start with feet together and hands on the hips. Twist the shoulders as far to each side as possible while keeping the hips still. It should be done in a slow and controlled manner.
Also, the NPA Mirror Drill could help. Start by getting into a strided position with with front foot opened as itwould be when you plant and hips and shoulders closed. Glide forward onto front foot a bit (so knee is over front foot) and then release the back foot and hips and rotate them as far forward as possible while keeping the shoulders closed. Hold that position for a moment and then rotate the shoulders and track the upper body forward while keeping the spine as upright as possible and the head and shoulders stacked vertically. This will let you practice separation and delayed shoulder rotation as well as tracking forward and rotating around an upright spine. You can do this drill with your arms crossed in front of you or you can go through your normal arm motion (both throwing and glove arms).