Chin Tuck Exercise
This exercise not only helps strengthen the muscles that pull the head back into alignment over the shoulders (upper thoracic extensors) but it also stretches the scalene and sub-occipital muscles.
Keep chin down so head is not looking up and push the head back.
The chin tuck exercise can be done leaning against the wall as well and at numerous times throughout the day, such as while sitting in the car or at the desk at work.
The repetition of this exercise throughout the day also helps develop good postural habits