Sitting at a desk all day can wreak havoc on your upper back muscles–the ones between your shoulder blades–but there is an easy way to take the stress off those muscles.
You can take little breaks to do shoulder exercises (protraction and retraction, elevation and depression, and arm circles) or while you are sitting you can think about your posture, trying not to let your shoulders roll forward and attempting to sit tall. Both are good options, but some people feel guilty or silly taking breaks to stretch during the workday.
When your back muscles start to ache, whatever you are doing (talking on the phone, in a meeting, working at the computer) take a deep breath and slide your shoulder blades down your back. Try to hold this position for as long as your mind can stay focused on it. Remember to breathe.