Also if you have a chance read Lower Back Disorders by Stuart McGill and colleagues. It is based in a lot of research but quite easy to read for non medical people. It's available on the net and there is a DVD based on it just out too. As a non physio exercise professional i found it very helpful.
On the work station note, there is research (forgive me for not having the ref on hand) that says that leaning back in the chair (as if putting your feet up on the desk) actually reduces disc pressure and that the 'best' sitting posture is one that changes often. McGill's book discusses this in more detail. Sitting puts a lot of force through the back so it is good to alter your position regularly and get up and stretch at least every hour.
Beware of lumbar flexion stretches 1-2 hours after waking up in the morning. Discs soak up fluid over night and are vunerable to injury in this time.