As humans we tend to focus primarily on what we can see in the mirror - our anterior (front side).
Taking some time out to work on the upper back muscles is a must.
Just observe the majority of persons sitting at their work desks or using cell phones. More often than not we present with rounded shoulders and a bent neck when doing so - both of which lead towards bad posture and aches if consistently done.
Counteract this with mindfulness when sitting or standing for that matter and also strengthening those upper back muscles such as the rear deltoids & rhomboids.