Level
Keep your back straight, sit slightly on the edge of your chair, with your feet flat on the floor. When you are playing a keyboard, make sure the instrument is at the right height for easy access.
Keep your forearm and hand more or less in a straight line. Keep your fingers a bit round, like there’s a soap bubble hanging from your hand. Your shoulders are just hanging. Keep everything relaxed, otherwise it will gonna hurt when you play a bit longer. Take a good look at pianist Martin Oei at his grand piano. Concentrated ánd yet very relaxed!
Thomas Bierman prefers standing at his instruments. In this way he can easily switch from one instrument to another. Do you see that his back is not bent either? He too keeps his a back straight. Playing a keyboard or piano should always be relaxed for you as well, and not too tiring.