Exercises to improve posture and stance can be found in the Alexander Technique and chi-running or chi-walking. The Alexander Technique was created by Frederick Matthias Alexander (1869-1955) and has benefits that go beyond posture and spill into other areas of your life. Chi-running/walking was developed by Danny Dreyer in 1999 and involves the use of tai chi in the motion of walking or running.
I'll start with chi-running/walking. The main concept is to think of your body as a needle in a cotton ball. The needle is your spine, and your shoulders and arms, and your hips and your legs can be thought of as cotton. You want your spine to be strong and your limbs to be loose and limber, not tight or stiff.
For your spine to be straight, think of a string being pulled from the bottom of your spine upwards through the top of your head and up into the sky. That is, stand up straight! As tall as you can. Try to make your head go as far into the sky as possible.
Danny Dreyer's book is called "ChiRunning: A Revolutionary Approach to Effortless, Injury-Free Running". It was the title that caught me eye the first time I saw it in the bookstore. Really? Effortless running? Injury-free?
He says that putting your body in the right position can drastically reduce injuries in runners. It's all about being loose and limber.
Now jump to the Alexander Technique. Think "up, out and wide". The "up" component is the same as what we just discussed in chi-running/walking. For "out", think of your arms being pulled out from your body. Try to make your hands go as far away from your torso as possible. Same things for legs. Try to make your feet go as far away from your torso as possible.
Too often we pull our body into ourselves. We tighten up into a little ball and shrink from the world. Like an animal goes into defense and curls into a ball when it's under attack. We need to stretch ourselves "out".
As for "wide", think of the left and right sides of your body going further out left and right. Pull your body out further left and pull it out further right, both at the same time. You want to be wide. You want your chest expanded and your head up. Think "walking like a marine".
These are the basic exercises to improve posture that the Alexander Technique and chi-running/walking offer. When you walk, try to think "needle in cotton ball" and "up, out and wide".
If you're interested in finding out more about the Alexander Technique, I recommend Missy Vineyard's book, "How You Stand, How You Move, How You Live".
The way you move your body, the way you "hold it" in space, has an effect on your health, on your confidence and on your mood. Amy Cuddy, whose TED talk on "power posing", has garnered millions of views, describes the importance of the position of your body in her book "Presence: Bringing Your Boldest Self to Your Biggest Challenges".
In the book she describes the "power poses" she introduced in her TED talk and how they can improve your confidence and mood. Yes, by putting a smile on your face, it can make you happier. Science backs this up. By putting on a confident walk, you can make yourself feel more confident.
These power poses are another form of exercises to improve posture and stance, although she mostly talks about the mood/emotional benefits from them as compared to the physical health/injury benefits.