Develop muscular endurance in the spine.
Do yoga or something that allows you to move your spine and stretch hip-flexors.
Sitting in one place is endurance work. Mostly our body gives way before the mind does.
You can apply other things suggested too but the physical component of focus is often ignored.
And of course develop the endurance to sit for long periods by sitting for long periods. 3 to 4 hours is usually ideal. (According to Cal Newport in Deep Work)
This is more micro than what you wanted but I guess it can work in a bottom-up manner.