After a little back pain, I made a few changes which make me feel a whole lot better generally, and make it much easier to get and stay in 'the zone':
- take breaks regularly
- stand up when not operating the computer
- stretch whenever you remember
- walk about
- exercise daily (as much or as little as you feel like)
These simple measures quickly become more of a joy than a chore, and will protect from back pain, RSI, hemorrhoids, fatigue, and probably a bunch of other nasty things that creep up on you.