Having a healthy back begins with proper posture. Among other things, proper posture:
So what exactly is posture? Posture is the position you hold your body in, whether it's while standing, sitting, or lying down. "Proper" posture is when you hold your body (while standing, walking, sitting, and lying) in positions where the least amount of strain is placed on the supporting structures like muscles and ligaments.
Here is some information from the Cleveland Clinic about proper posture while sitting.