Did you know that 99% of all Headaches are Posture related? Tight upper back and neck muscles cause nerve compression and lack of blood flow which leads to headaches.
This is a simple behind-the-back stretch. You can do it sitting or standing. If you're going to sit, make sure you have a chair with backing that comes halfway up your back. Hook your arms over the back of the chair and pull your shoulder blades together (if you're standing, assume the military
Now pull your shoulder blades together until you feel a pull in your shoulder joints. Don't go overboard - you don't want to cause any pain.
I want you to hold it for three full minutes (don't cheat and quit early because the length of this stretch is very important). Then slowly release the stretch.
I suggest you take breaks throughout the day to perform this stretch....it will help a lot!