I sit down and I adjust myself to the point of highest spine, widest shoulders, and calmest neck. I soak it in for a moment, then I continue with the conversation, people, or scene around me. As I divulge into the present, every so often I find myself losing that stand, posture, and that solid being that I initially created. So I move back into it. It helps to squeeze your shoulders together throughout the day, or push them up to your ears and roll them back down. I like to stretch back, sideways, or cock my head back. Anything works when you just move it dramatically in order to find that perfect balance.
DEN OF THE LIONESS 
I recently read in a yoga book, that a little bone called the hyroid bone, situated in front of your neck at the back of your jaw has a lot to do with posture and from what ive been experiencing it really helps.
Take a moment to feel your body (like you said) and lift your chin up; your belly expands and shoulders go forward, but when you tuck your chin in the belly sucks in and the shoulders blades go back, collar bone rises. So, keeping your chin in a comfortable, just slightly tucked, position is best for a good posture. you might look funny though if you over do the tucking your chin. This is because that little bone pretty much holds up the tissues of your stomach lining, like a bag hanging from a nail in the fence. when you tuck your chin the stomach rises and your shoulders and chest make more room for your lungs.
The body is always compensating for our posture (not always efficiently) as it makes space for the organs especially the lungs. So to have good posture like when practicing yoga you want to feel what adjustments create the most space internally, listening within to let your organs guide the posture. Having your chin slightly tucked in certain postures (like standing stick and boat pose) helps keep the spine aligned but also, helps tone and strengthen the tissue supporting the front of the spine.