Chad Waterbury put up a
short little blog post on T-Nation containing a good thoracic spine mobility drill. It promises the usual instant and amazing gains, but even if you don't get them you'll improve your ability to move around the upper back. That's good, because the lower back's role is to be stable and provide a good base of support while the upper back moves. Stereotypical computer users hunch over their keyboards, slowly but surely making their upper back unacceptably "stable" (read: immobile) and forcing their lower backs to flex. This leads straight to back pain.
So give this one a try.