Aim to situate yourself comfortably in a stable, balanced position so you can play and read music, with room to move freely. Keyboardists can get so caught up in the complication of notes, clefs, fingering, rhythms, and dynamics that they often forget about their bodies.
If you're too close to the piano or keyboard, you cramp up your arms and scrunch your shoulders. If you're too far away, you overreach, putting stress on those parts (the neck, upper arm, and pelvic support) that have to work to support your overextended parts. You want to feel comfortably loose, with enough room to move freely and enough support to feel light and long in the upper body and head. You should be able to move easily in either direction of the keyboard, just enough to follow and support your arms when they venture away from the middle to the high or low registers.
Check that your bench or chair is the right height. A common way to measure this is by seeing that your elbows are even with the height of the keyboard when you're sitting at the piano with your hands in playing position. You should see a slight arc from the elbow to the top of your wrist and back down your hand to the keyboard:
Good posture is all about support. Think about building support from the ground up:
With the floor and the bench or chair providing your support base, align your body so your torso, shoulders, neck, and head are fully supported from underneath.
With both feet on the floor, and with your knees directly above your feet, sit evenly on your sitting bones so you feel a strong, stable support for your upper body.
Don't let your weight fall back; bring the back of your pelvis (your hip bones on the sides and sacrum in the back) up above the sitting bones, and continue this line of support up through your spine to the top of your head.
Your spine has four curves; it takes careful awareness and support in both the front and back to balance and feel centered throughout your upper body.
Keep your head supported above your spine; don't let it drop or lean in any direction.
If you're hunching, slouching, or leaning, you're going to have to use your muscles and energy to compensate for the imbalance.
Release your shoulders if they're holding any tension, and let your arms hang to the side.
As you breathe in, feel the full length of your upper body from the sitting bones to the top of your head.
Breathe out and feel a relaxed, stable balance throughout your body.