The foundation of good posture involves what is translating your weight in to the earth.
If you're sitting, you'd better make sure that your ass is grounded ( and not too tight).
If you're lying down, pay attention to what has contact, and where are the spaces; you'll learn a lot.
And if you're standing or walking, well, it's your feet.
In many cases, humans don't use the entire sole of their foot, and that can cause problems not only in the foot, but in every joint higher than it ( so, basically, every joint in the body).
This short sequence gives you tools to re-align your feet, and makes your legs feel great while you're at it.
I'd also like to give a shoutout to Jenn Pilotti, whose insight in to my feet changed me forever. She also has a studio in the Barnyard, and often teaches workshops on the weekends. Check her out.