Use an adjustable seat and ensure that your feet touch the floor.
Sit on the edge of the piano bench and avoid taking up more than half the bench. The majority of your weight should be forward and directed toward your feet.
Place your knees just below the piano keyboard. Your thighs should never sit under the keyboard as this will create tension in your arms.
Stretch your arms before playing by placing one arm behind the neck and straight into the air. Grab the straight arm with your free hand to make a T-shape with your arms.
Position your elbow closer to the instrument than your shoulder. The elbows should be about 3/4 of an inch in front of your shoulders.
Align your forearms so that they are parallel with the floor. This may mean you have to further adjust the piano bench.
Straighten your back to ensure that you don't put any stress on the middle of your back.