Restriction in the upper/middle back, or thoracic spine (T-Spine), is one of the most common areas of the body to tighten up. And it's insidious, too. That is, the tightness evolves slowly month by month and yet you may not even realize you're tight there. Instead, you'll feel your shoulders get cranky, your neck get stiff, your elbows become prone to tendonitis, and even your wrists can pay a price. Carpal tunnel anyone?
And we didn't even talk about the lower body yet!
In gait, we see rotation in many places. For example, the pelvis rotates to one side and the shoulders turn the opposite direction. Now imagine you've lost the ability to rotate something. In this case, if “t-spine” rotation is lost, then we are forced to compensate for that my moving excessively somewhere else.
In my world, moving excessively sounds a lot like instability. By that, I mean you'll give up your natural stability somewhere else if you are too tight. Commonly, shoulder blades become unstable or, due to t-spine tightness, your low back will compensate by becoming less stable.
As outlined by Mike Boyle and Gray Cook in the Joint-By-Joint Theory, certain areas of the body are prone to becoming tight and certain areas are prone to instability. The T-Spine is definitely prone to tightness since we spend so much time ...wait for it... sitting.
Furthermore, I believe being stressed while sitting is truly the killer. 
I'm sure you're never stressed while driving or stressed while seated at work, right? Stress creates tension and a hunched position. Do this hours a day, day after day, year after year, and no wonder our thoracic spines get jacked up!
Whether you prefer to run, lift, or walk, you need enough t-spine mobility so that you don't compensate for a lack of it. You don't need to be a contortionist, you just need 'enough'. Even regaining a few degrees can make a huge difference in how you feel. So don't underestimate the following two mobility drills.
Kneeling T-Spine Extension
http://www.youtube.com/watch?v=XWr63-HpRsc
Side Lying T-Spine Rotation
http://www.youtube.com/watch?v=MmtjmT1keck
I utilize diaphragmatic belly breathing to facilitate better relaxation during these stretches. In fact, I don't even think of them as stretches. We're not stretching or yanking anything. We're getting into a position and relaxing. Proper breathing allows your muscles to relax into a longer length and, FYI, your entire body works better when you're relaxed. Hmmm.
A super, fantastic, take-it-to-the-next-level idea would be to perform a movement sequence like downward dog – chaturanga – upward dog immediately after the loosening drills. They will help to program your body to use and retain that new mobility. Of course, you could do my other favorite move, the Turkish Get Up for excellent results, too.
Finally, the obvious: stand more, sit less, and simply be aware of how you are positioned throughout the day! What's your middle back doing right NOW? :-D
Try these out and let me know how it feels!