Posture is not about holding a static position and sitting up straight.
Although we tend to think of posture and movement as distinct entities, posture in fact consists of constant subtle movement. Subtle movement and variation helps maintain comfort, balance, and preparation for whatever movement comes next. The better you move, the better you can just sit and relax.
Here are some articles on posture, several of which challenge some common myths, including that static posture is a major cause of pain, or that it requires sucking things in, pinching things back, or holding different body parts in place.