The Rotator

http://www.exrx.net/Articulations/Shoulder.html

Which movement are you talking about? If your talking about external rotation, as in if you are holding your arms out straight to the side, bend your elbow down so the hand points towards the ground, then rotate so the hand points towards the sky, then this might help out as a static stretch.

http://www.exrx.net/Stretches/Subscapularis/Broomstick.html

But, i find that dynamic movements stretch better than static. holding stretches haven't been proven to increase functional flexibility. They are good for shutting down the muscles to let them rest and recoup. There's a place for static stretching, and dynamic stretching.

Dynamic stretching would be just performing the movements to get the muscles involved. If you never used the muscles before they are going to be tight. It like someone who sits all day, there hamstrings get use to being in the bent position, so they get back pain because there hamstrings are tight. If you start doing some dead lifts, just using body weight or some light dumbbells, you can increase the flexibility way more than if you try to do normal hamstring stretches, like touching your toes. Like most people who maybe don't work out a lot or don't focus on an area of the body. Just start lifting light weights, learning some of the basic complex exercises. Dumbbells are great. So are Resistance Bands.