a simple strengthening routine can improve the stamina of these muscles, improving posture and reducing discomfort and triggerpoints in the upper back and shoulder girdle. there are many ways to accomplish this, but the real trick is finding a 'tool' you can fit into your regular routine, without a lot of extra gear.
here is a simple one that can be done at home. i really like the arm rotation aspect of this exercise, as it helps to open up the chest as well.
the act of simply pulling the shoulder blades towards each other while standing (in line at the grocery, waiting on hold for your cable tech representative, etc.), and holding the contraction for 3-4 nice slow breaths is also an effective way to fire those muscles and build better posture. try starting out with 2-3 sets at a time, once or twice a day.
weight/resistance training is a great tool as well, and i'll continue to add tips and links here...
