Keep tension at bay by moving. We are taught to sit like statues and after years of playing in postures that are sometimes required by our instruments, asymmetrical body development may occur. Muscles become tight and weak on one side and strong on the other. Static postures are the enemy. Muscles are not replenished with oxygen and tire quickly. Even onstage one can move unobtrusively. Here are just a few suggestions.
Take every opportunity to allow your arms to uncurl and hang down for a moment. Shrug or roll your shoulders. Release thumbs and fingers as often as possible. Don't hold stretches, chords or awkward positions any longer than necessary. Avoid grabbing or squeezing your instruments. Do circles with your wrists and thumbs. Press your shoulder blades together stretching your pectoral muscles. Reach one or the other arm behind you and if you can, place your hand flat on the chair behind you. Tuck in your chin and gently look left, then right. Tuck your chin again and move your right ear gently to your right shoulder, then left ear to left shoulder. Take a deep breath and as you exhale pull in your abs. Adjust your seating and move your feet. When you stand to take a bow reach one arm behind you bending your elbow as if you need to reach an itchy spot near your shoulder giving your pectorals and your shoulder a stretch. Try standing on your toes, squeezing your buttocks and rotating your hips.
There are many more stretches and moves described in Playing (Less) Hurt for both onstage, backstage and at home. All of these small movements are tension busters and your muscles and tendons will appreciate these mini-breaks.