I have lousy posture, so bad that it causes the back of my neck to tense up, giving me back headaches. I went to physical therapy for it and they gave me a bunch of exercises to help. Most involve flexing my shoulder blades toward each other, or pushing my shoulders downward. One has me bracing my hands on either side of a doorframe and pushing my body inward (almost through the doorway). They do help alleviate the headaches, when I actually keep up with the exercises.