Plant two feet on the ground.

Drink plenty of water. If you are in a library keep it in your bag. No need to make guards nervous.

Extend your spine from time to time.

Enjoy your pen.

Select paper with optimum texture, weight, and smoothness, something which feels good to the touch.

Take breaks for tendon gliding exercises

Embrace errors. Cross them out and keep going.

Walk around and stretch your legs every 45 minutes or so.