Diet and exercise, especially stretches.

Massage has helped me immensely, especially since I found an RMT (registered massage therapist) who knew hands. To a lesser degree for me personally, physiotherapy, as it was focused on my shoulders.

Playing posture, classical with all fretted instruments, has helped with comfort, lessening muscle tension.

Practice regime, as in frequency being more important than duration. Several short periods of practice is better than one long period.

Technique with a light touch.

Gear that works for you not against you in scale length, string gauge, set-up, proportion and size.