I had some pretty unpleasant residual tension in my pyriformis after a pinched nerve, and learned a few ways to deal with it.
1) Epsom salt hot water soaks. Gets the job done.
2) Sit, lie, stand, whatever, with a tennis ball or lacrosse ball under the sore spot. The more it hurts the better it feels. You'll feel the muscle spasm then release when you get it just right. It takes about 5 minutes.
3) The spot that hurts might not be the spot that's hurting. Referred pain is difficult to treat sometimes. With pain in your middle back, it could be caused by anything from your jaw, to your neck, to your shoulder, to your back. Pain tends to move downwards and inwards. Stretch (and strengthen) your neck, make sure you don't slump your head forwards, and triple check the ergonomics of your workstation.
4) I'm not a big believer in the term 'psychosomatic' because it sounds too much like 'all in your head.' Look for times when you're stressed, and run a quick check through your body for tension. If you find it, flex the tight muscles for a few seconds and release.
If you find yourself tense often, and I truly mean this in a respectful way, you have to deal with some important parts of your life. Persistent pain can be your body's way of telling you to address something.