What about lying down? A few years ago, BBC covered a scientific study[1] showing that sitting at a 135-degree angle dramatically reduced spinal disk movement. Anecdotally, I found that this sitting position, midway between sitting and lying down, is most comfortable for prolonged periods of time. Maybe at my next workplace I'll insist that my office be fitted with a lounge chair.
http://news.bbc.co.uk/2/hi/6187080.stm