Does the military really recommend this for standing at attention? Perhaps Ms. Pryor, Brian, or Jim will tell me. It's actually not good posture for someone who might need to move into action quickly.
Why? By throwing the shoulders back and sticking out the chest, you are not allowing your core muscles to be engaged. They aren't helping to keep your back straight--it's going to get tired. And your core muscles--the muscles of back, abdomen, and laterals--should be the epicenter of every movement. If movement starts from the core--or from the belt, as we like to say in martial arts--it will be stronger, more graceful and balanced.
Taking ballet has helped me improve my core muscles and has given a core of strength to everything I do, including TKD. In ballet (and in Pilates, I think), we learn to stand like this.
Do you see how her shoulders are right over her hips, and hips over knees, and knees over feet? That's good alignment. She's ready to move.
To achieve this, I think about aligning all those levels of my body, stacking one over the other. And then I think about my core: Carol says "short in front, long in back." You don't so much pull in your stomach as you close the rib cage and feel your ribs pull gently down toward the navel. Most people will need to tip their pelvis forward (lengthening the back).
It's easier to do this if your core is STRONG! I find this position rather unnatural; when I've been out of ballet class, my posture gets worse quickly! But when I'm doing ballet, it becomes more natural.
If you're not taking ballet, you can improve your posture by taking Pilates. Or you can do curls (modified sit-ups: press your lower back into the floor and initiate a small, curling upward movement with your abdominals)--maybe start with 10 and work up. I always like to start small when it comes to exercise!