There are "posture experts" everywhere that teach you to how to stand: bones in alignment, body parts stacked just so. Many yoga teachers stress alignment more than they do somatic awareness and proprioception. Because most people have Sensory Motor Amnesia and don't know it, it's even more important to understand how our brains control our muscular system as a whole and how stress reflexes create a distorted internal sense of how our body is connected, how our joints move and what it feels like to stand squarely on our feet. One of the worst pieces of advice people are given is to "stand up straight!" One of the least helpful opinions about "why" people have poor posture and back pain is "the back muscles are weak." I am a former professional dancer and many of my teachers had intractable back pain (and retired early) while having extremely strong back muscles.
When I ask people to stand up - and sit up - to what they think is "straight," they typically arch their lower back in an effort to pull the shoulders back and open the chest. I see this in yoga class as well. This posture - a strongly arched lower back and tight shoulders - is called the Green Light Reflex (or Landau Response) and it is a major cause of chronic low back pain.
Life is dynamic - so are you without back pain
A healthy body is one that can adapt and adjust to whatever feedback comes in through the environment, yet can find its way back to balance and relaxation. Yes, life is dynamic, as is efficient, functional posture. Just like the ladies in the photo at right, balancing life and balancing books requires the ability to find center naturally as you move.
Many people work really hard to "get the right posture" not realizing that they're actually tightening and bracing certain muscles in an effort to attain it. Again, this contributes to back pain as well as neck pain, shoulder pain and hip pain. What would it feel like if you learned to let go of muscles that are unconsciously tight and tense in order to find your "perfect posture?"
Achieving good posture is about learning to relax muscles that aren't crucial to holding you up, while allowing the muscles that need to work to coordinate together in perfect balance and ease.