I do stretches to open up the muscles in my chest, shoulder area. Most are from my former yoga trainer. I also lay on a big foam roller (not to be confused with the hair foam rollers). 
My tailbone at one end and my head at the other. My feet are on the floor with bent knees. I slowly extend my arms to the side, shoulder height, on the floor. After I am comfortable here I slowly bend my elbow, arm still on floor. I feel the stretch in the front of my shoulder and in my inner upper arm and in my chest. I stay like this until I can feel some of the muscles relaxing.
I also am always reminding myself to relax my shoulders. I can do it now but before I started stretching a lot I had a hard time relaxing them. I do stretch everyday. My posture is much better when I am consistent with my yoga, Pilate's and slow stretching.