1. Lie flat on a bench holding light fractional plates. (These are specialty plates that are smaller than even standard 2.5 pounds. Any another light weight will work too.)
  2. Depress your shoulder blades down. Tighten your abs. Try lifting your knees to your chest to encourage a posterior pelvic tilt.
  3. Hold the plates straight up above your chest. Keeping your arms locked out, lower the plates down behind your head.
  4. Drop your arms as far as they go without bending and begin to relax and hang out here. If you have any pinching in your shoulders stop immediately and go see a professional.

Focus on relaxing your lats in this position and begin to move your arms, replicating pressing and pulling movements. Try straight up, straight back, up and down, etc.

If there's a noticeable difference in either arm, see if you can start to balance it out. If you don't feel like you're getting much of a stretch and you have no pain, then you could try heavier plates.

As for times/reps etc., just do it until you have a noticeable improvement. Always have something to retest afterwards like your overhead press.

If you're a big guy and this gives you no improvement, you could also include a roller in the stretch to increase your thoracic mobility. Just treat this exercise with respect. It's a stretch, not a strength exercise.

Related:  The Very Best Low-Body Mobility Exercise

Related:  Tip: One Move for Indestructible Shoulders