Aside from being aware of your posture and changing bad habits you can assist your body to improve by strengthening muscles that have become stretched and weak over time and stretching those muscles that have become strong and too short over time.
The chest muscles have become too short and tight.
The trapezius muscle which covers the area from your head, upper shoulders and back may best be known as a source of stiffness across the top of the shoulders.
If you sit at a desk for long periods of time, the muscle becomes overstretched. When this occurs the muscle contracts to try and pull your body back into correct posture, which ultimately ends up straining the muscle. Initially feeling of tension will start between the shoulder blades which can increase to feeling of soreness and pain that can radiate from the shoulders to the back of the neck. This pain can increase to a constant ache that may be very hard to relieve.
To strengthen this muscle, lie on a mat or exercise ball face down. If you are lying on a mat, use a small towel and place under your forehead. If you are lying on a ball, look to the floor and try to keep your neck in a neutral position. With arms extended out in front of your shoulders and thumbs pointed to the ceiling, slowly lift them up until you feel the contraction of the muscle between your shoulder blades.
The focus is to improve the posture by increasing the muscle endurance. Endurance training has a more positive impact to improve the function of the muscle. Endurance training is done by performing an exercise with lower weight and by performing a large number of repetitions over a prolonged period.
There is always a variable when determining how many sets you do and how many repetitions of an exercise you should perform. The exercises should never cause pain, so if you experience any pain when performing the exercise, discontinue and consult your physician, massage therapist, trainer,……
If you wish to improve muscular endurance, the National Academy of Sports Medicine recommends that you perform one to three sets of 12 to 25 reps at 50 to 70 percent of your maximum effort. (http://healthyliving.azcentral.com/workout-trapezius-20506.html)
Start with one set (1 time) and do as many repetitions as you can before you fatigue. Repeat every other day.
As you get stronger add another set (perform as many reps as you can, rest briefly and perform the reps). Continue to add another set. When you can complete 3 sets with numerous repetitions without fatigue you many increase the weight or continue to maintain the endurance.
If you have a job that requires you to continually put your body in a position that creates bad posture, you may want to consider performing your exercises for an extended period of time. (weeks, years…)