Posture: straight joints, loose shoulder

What really hurts over time is bad posture — above all the sharply bent wrist of the fretting hand. Keep the wrist as straight as possible, the thumb behind the neck as a counter-bearing, the shoulder loose, the guitar close to the body rather than stretched out front. If you practice seated, set your foot or the guitar so the neck doesn't tip downward. A neutral joint position costs conscious control at the start and saves a lot later.