My t-spine is pretty inflexible. I've been working on it, and my trainer gave me some correctives that I've been careful about doing. One side is greatly improved, the other side needs more work. I've only recently learned that our mobility actually comes from the t-spine, it isn't supposed to come from our lumbar spine!
Outside of the correctives I've been foam rolling it, and doing pretty much everything that I can think of/find information on that doesn't feel like it will make my spine come out of my mouth. I've read that taping 2 lacrosse balls together really helps to dig into the gnarly bits, and I tried that this morning at my gym before group training. I certainly found a nasty little area just south of where I feel most of the tightness on the left side of my t-spine and I think I need to have a set for home use.
If you have the same issue and have found exercises/stretches that have worked for you please post them. I'm sure it is from too many sedentary years hunched in front of the computer. I think I need to add at least a couple of these to my post-ride stretches as well though my riding position isn't as aggressive as most.