This one exercise can greatly improve your mobility and flexibility -- and cure the many ills that come from sitting hunched in front of a screen all day. It takes just 30 seconds, and you don't need any equipment.
Max Shank teaches us how to do this move, called the Thoracic Bridge. It basically opens up your body and can help with pain and tightness in your shoulders, back and hips. It looks a little weird to do, but when you get the hang of it, you should automatically feel the difference in your body's alignment and posture. (Also, you don't have to be as crazy acrobatic as Max in switching sides.)
There are lots of easy stretches you can do to correct your hunching and get all the benefits of better posture, but if you're only going to do one thing, the Thoracic Bridge move is arguable the best one, since it corrects so many issues in half a minute.