How can i help a stiff neck caused by poor posture?

Many ways: The first thing to do is to understand what good posture looks like. There are a number of resources for this but probably the best way to learn to take control of this yourself is to have a few sessions with a physical therapist who can teach you. Postural exercises looking in a mirror can help. One of my favorites is yoga which spends a great deal of time working on alignment.

Better head position: If your neck pain is truly being caused by poor posture, then improving your posture is the obvious answer. The usual neck posture problem is keeping your head in a too forward position. Try positioning your head further back with your ears centered over your shoulders and your chin tucked in a little. This positions your head in properly for your neck and often helps the neck stiffness.