Here's a (hopefully) easy-to-understand concept for keeping your posture both right and relaxed.
Imagine that your spine is a coat stand. It is straight up and down, solid and grounded. Try to imagine that your head is being gently pulled straight up toward the ceiling, with the crown of your head at the highest point. Sit squarely on your rear, centered on your seat/throne. You should be able to
Next, your shoulders are a coat hanger hanging on the coat stand. Again, gravity pulls the shoulders/hanger down, but they are free to move and swing around on the hook on the coat stand.
Finally, your arms are a shirt hanging on the hanger. Gravity is still working, no?
Your arms are free to move about, but no extra stress is put on the muscles of the back or shoulders. The main muscles that hold your arms in proper playing position are the biceps.
Focus on being as relaxed as possible, and using the most efficient muscles to get your arms and hands into playing position. You may find yourself adjusting your drumset to conform to your newfound posture and relaxed-ness.