Stand with one arm against the corner of a wall or the edge of a doorframe. Using the edge push the head of the humerus bone backwards (this minimises the stress on the anterior capsule) and turn your head in the opposite direction to intensify the stretch.
Retract (bring shoulder blades together) and depress (lower towards the ground) the scapula (shoulder blade).
Another way to stretch the pectoralis major and the minor, is to move your arms up the door frame and push through, retracting and depressing with each push through.
This is another quick and easy pec stretch you can do anywhere, no wall or doorway required!