The following are youtube videos featuring my top 3 favorite poses for opening the shoulders.
1. Yin Yoga for the Shoulders: 1/4 Dog Pose. This is really good to relieve shoulder pain and upper back stiffness. Helped me a lot when I had shoulder injury.
2. One Armed Shoulder Opening. This is by far my favorite for relieving pain in the shoulders. And a great shoulder opener too!
3. Yoga for Opening the Shoulders. This is not exactly a pose since it is a sequence. This is my favorite sequence to open the shoulders. It aims to increase mobility of the upper back, chest, and shoulders. Great for improving back bends. I used to do this everyday for around a month and I was able to increase flexibility in the shoulders, chest and upper back.