When I was in my 20's I had the coolest most awesome piano teacher. Here is what I learned that saved me years later when I got a house gig playing 4 hours every day.
Important! Adjust the height of your bench or (if you play standing up) your keyboard, so that your wrists are as unbent as possible. if you sit too high, your wrist is bent with knuckles above your wrists (picture a cat trying to strike with its claws). Too low and your wrists are bent the other way (picture a dog sitting up and begging.) In either of those cases your tendons have to go around a corner to pull your fingers closed (which is the keyboard players main action.) The resulting inflammation of the tendons can become chronic and you will have to quit for a long time.
In addition, try to relax the arms as much as possible especially when playing loud, fast, or most especially when playing loud AND fast. Use relaxation and the weight of your arms to create volume, not the tension of a rigid arm striking. This is the condensed reader's digest version of most basic Arrau DaSilva technique. The most natural ergonomic hand motion is to grasp. if you use the grasping action instead of the striking action, you can eventually play all day without injury from impact. If you strike something while your arm is tense you create the most classic scenario for "Tennis Elbow" or tendonitis. This begins with a tired low-energy feeling in the hand and arm motions. Rest and relax!