Matt Mast demonstrates a great shoulder stretch with a wooden dowel.
With a wide grip, take the dowel from in front of you, to overhead, and all the way behind, without bending your arms. Once you can do this comfortably, close your grip slightly and bring the dowel around again. Continue this process and stop short of the grip where your shoulders are no longer flexible enough to rotate through the full range of motion with locked arms. We like to determine overhead squat grips or snatch grips in this manner.