Ideally, you should sit with a straight back, supported by the chair, and possibly a cushion at the bottom of the spine. If you stay in one position for too long, muscles tend to stiffen up and become strained. If you have to write a lot, it's best not to use a flat desk because it puts a stressful curve in the back. When you walk, try to keep the top of your head lifted. And , don't slouch.
Stress too is a frequent source of back pain, because it makes the muscles tense.
Try to make a conscious effort to relax. Attending an Alexander Technique workshop or seeing a teacher will help you improve posture and show you simple ways to not wear your body out. Lifting and bending Lifting is one of the most common ways of hurting your back, and you don't have to be lifting something especially heavy. If the object is heavy, always get help. But whatever the weight you need to use the correct technique.