One hundred and fifty years ago, daily living required us to perform tasks that worked our entire core equally. This resulted in equally toned front, back, and side torso muscles. Therefore, good posture occurred naturally. Now youth and adults are required to do repetitive movements such as small strokes on a keyboard. As we sit at computers all day, our shoulders hunch over causing the front of our shoulders and chest muscles to shorten while the back of our shoulders, neck, and back muscles all elongate. Having our legs bent under a desk then shortens our hip flexors and psoas muscles which pull our lower back out of alignment. All of these things affect our posture.
So how do we maintain good posture (or gain good posture)?
First, we need to exercise. All exercise is great, but weight-bearing physical activity not only helps build muscle, but also helps improve bone density which will help improve posture. Be sure to add in core exercises as well. For sample core exercises, visit ACE-fitness http://www.acefitness.org/workouts/5/
You can also begin making small changes such as adding short stretching breaks throughout your day. Or consider replacing your comfy chairs with stability balls!