Adjust the backrest of your office chair so that the lumbar support rests in the curve of your low back (about where your belt line is). Support is needed in that spot so that your vertebrae stack properly on top of each other. When you are standing your back maintains a natural curve, but when you sit down this curve flattens out and places extra and non-uniform pressure on the intervertebral discs. This pressure places stress on the soft tissues of the back and may result in discomfort and possibly injury. If you do not have lumbar support in your backrest, a small folded towel will also work.