Prolonged static sitting is a frequent aggravating factor for individuals with spinal pain. The single best advice that we can give as a PT is to tell them that they must frequently interrupt static sitting, ideally at least every hour.
Relative to the cervical and lumbar spine, thoracic symptoms are less common. However, if someone reports of mid-thoracic discomfort related to prolonged flexion and sitting, it is likely that they have become sensitized into flexion. Therefore, self-mobilization into extension is the most logical way of desensitizing the thoracic spine.
Simply lift the arms up overhead 5-10 X every 1-2 hours, in order to interrupt the static posture and desensitize the spine.