The insides of your feet should be as wide as your shoulders and your weight should be centered left and right (Half your weight should be on your left foot and half on your right) and also from heel to toe.
Your knees should be flexed slightly allowing for balanced movement in any direction. With the proper flex, your knees should be directly over the arches of your feet and you should feel completely balanced and light on your feet.
You should be bent slightly forward from the waist creating an axis on which your shoulders can turn. Once you bend forward from the waist, your shoulders should be positioned over the arches of your feet. This is a very balanced position.
Think of a football quarterback under center and try to duplicate this position as closely as possible. Your weight should be centered on your feet not only left to right, but also from heel to toe. Your knees should be flexed slightly allowing for ease of movement and you should be bent slightly forward from the waist creating an axis on which you can turn your shoulders. Your arms should hang naturally from your shoulder sockets in a very relaxed fashion.
As hard as it is for me to understand, just like the grip, posture is often overlooked by many people who are learning the game of golf and also by many people who are struggling with their games. They go out of their way searching for the 'silver bullet' when the real silver bullet is hidden in plain view.
Quality posture is not hard to learn but it does require focus on the part of the player to maintain. Ask any successful touring professional how often he or she works on his or her grip, posture, and alignment, and I'll bet you the ranch, more often than not, they'll answer 'daily.' Ask the same question to a beginner, mid, or high handicapper and I guarantee you won't get the same answer.
Without proper posture as with grip and alignment, it is nearly impossible without major championship talent and a ton of time to practice to consistently hit quality golf shots. Golf is so much easier when you develop solid fundamentals and I hope you'll take the time to develop them today.
If these fundamental concepts carry enough weight that the top touring professionals are focused on them on a daily basis, then I think that you and I should be focused on these fundamentals as well at least every time we get to practice and play.