Poor posture creates a lot of stress on your spine and chronic muscle tension, leading to pain and stiffness. This happens because the weight of your upper body must be supported by your back muscles instead of the bones of your spine. The further your posture deviates from normal, the greater the stress on your back and the more pain that you will experience.
To illustrate this idea, think about carrying a briefcase. If you had to carry your briefcase with your arms outstretched in front of you, it would not take long before the muscles of your shoulders would be completely exhausted. This is because carrying the briefcase far away from the center of your body places an undue stress on your shoulder muscles. If you held the same briefcase down at your side, your muscles would not fatigue as quickly because the briefcase is closer to the center of your body and the weight is, therefore, supported by the bones of the skeleton, rather than the muscles.
A simple exercise to improve your posture is called the Wall Posture Exercise. To do this exercise, simply find a flat wall. Rest your back against the wall and scoot your heels, shoulders and head back until the touch the wall. Holding this posture, step away from the wall and try to hold this posture as long as you can while you go about your day.