Posture you don't have to think about
Good posture is not a position you hold. It is a habit you build in three places, until standing well becomes the thing you do without deciding to.

Everyone has been told to stand up straight. Almost nobody has been told how, which is why the advice lasts about ninety seconds. Standing straight is not a position. It is the result of three small habits, and once they are habits, you never think about them again.
Start from the ground
Posture is built from the feet up. Stand with your weight spread evenly across both feet, slightly more toward the heels than you would expect. Let your knees be soft rather than locked. From there, imagine the crown of your head being drawn gently upward by a thread. The spine lengthens on its own. The shoulders, which everyone tries to force back, simply settle down and slightly back when the ribcage lifts. Forcing the shoulders is what makes posture look like effort.
The phone problem
The single biggest posture habit of our era is the forward head. Every time you read a screen, your chin drifts forward and your upper back rounds to follow it. The fix is not to stop reading. It is to bring the screen to your eyes rather than your eyes to the screen: hold the phone higher, raise the laptop, and keep your chin level. Do this for a week and you will feel the difference in your neck before anyone sees it in your bearing.
Sitting
Sit toward the front half of the chair with both feet on the floor, or with the ankles crossed and tucked slightly to one side. Keep the same lengthened spine you found standing. Lean forward from the hips when you want to engage with someone, not from the shoulders. Sitting well is the posture most people see you in, at desks and dinners, and it is the one most people neglect.
Walking
Walk from the hips with the head level and the eyes at the horizon, not the pavement. Let the arms move naturally and keep the shoulders quiet. A steady, unhurried pace reads as confidence in every culture. If you wear heels, shorten the stride slightly and let the heel land first; the walk becomes smooth rather than stamping.
A two-minute practice
Twice a day, stand against a wall with your heels, hips, shoulders and the back of your head touching it. Breathe there for a minute. Then step away and keep the shape. That is the entire exercise. Within a few weeks, the wall is no longer necessary, and the posture is no longer something you do. It is something you have.
In the Signature Course we rebuild all of this in person, with a camera running, so you can see the change rather than take my word for it.