Take breaks to rest your back! Humans aren't "designed" to hold the same position for hours on end. Even short (20 second, even) breaks can be extremely helpful:

Lie on your back. Knees bent / legs up, so your back is fully supported by the floor. Arms at your sides, hands on your stomach or whatever the most neutral position is. Small pillow / book under your head to keep that portion of your spine properly curved.

Source: family member is an Alexander Technique teacher, also personal experience.