How to keep your back healthy working 8 hours sitting at the office

First thing is to adjust your chair and desk properly, with the monitor at eye level and your elbows at a 90-degree angle, because bad posture from the start is going to wreck your back. Every hour or two, get up, walk around a bit and stretch your shoulders and spine, even if it's just five minutes, because sitting still all day is the worst thing you can do. If you can, invest in a decent chair with lumbar support, but honestly what helps the most is moving around frequently and not assuming that sitting properly is enough.