Sitting is and leads to problems with pain by encouraging bad posture. There has been a movement toward standing desks and the like which presents a list of other problems. However, moving between sitting and standing throughout a traditional work day will help. But, we must embrace the truth, you need to reset your body and one of the best ways is through postural training using yoga and stretching methods. Regardless of what you might read on the internet, in my 20 years I have seen personally and know a number of other professionals whom have been at this game longer than I see tremendous success in using and applying yoga and stretching principles.
Here is a guide to some great postures and a great start. Most important is to do them and do them daily. Do not stress over time, etc. just practice and all will come especially if you stay consistent.
The following document includes some basic and effective stretches for the Shoulders. Included is variations for differing levels. As always there are additional things that could be added in to any health shoulder program. One is simply adding these into your daily regimen of stretching. Another is doing joint mobility drills such as rolling the shoulders, swimming motions, etc. that take the joint through active and various ranges of motions. Third, one can add in simple strengthening exercises from pressing, to side/lateral raises and more commonly rotator cuff exercises.
Hope this guide helps and if you have any questions please feel free to email. Also, please feel free to share this guide with anyone you feel can use these stretches.