Having strong, confident, excellent body language isn’t just about having good posture for posture’s sake, or making your mother happy. Non-verbal cues are important: people instinctually use them to size up your attitude, personality, and state-of-being. If you currently walk around bent over, or constantly looking down, or hunching against yourself, it can communicate things you may not want. Some people do this to make themselves look smaller, like very tall or very shy people.
The “proper” way to walk around, we are often taught* is back straight, chin up, eyes forward, chest out slightly, smile on your face.
*This advice is intended for any secular business or social setting in North America. Your situation may vary.
However, just being told to stand up straight doesn't really cut it. To help, I share two “metaphors” that you can visualize in your mind to help you adopt good posture. Pick one:
First metaphor: this is from How to Talk to Anyone: 92 Little Tricks for Big Success in Relationships, by Leil Lowndes a renowned expert of interpersonal tactics. When you are standing up and walking around, imagine you are in the circus. There is a trapeze or iron jaw hanging above you: a rope holding a bar or leather bit and hanging it just level with your mouth. Now, imagine you bite it, and remember that the trapeze or iron jaw is just barely at the level of your mouth. So you are biting into this thing that is forcing you to keep your mouth level, maybe even imagine that it is pulling up on your mouth a little. Doing this will help you have good posture.
Second metaphor: I heard this alternative trick from a Jazz Musician. Stand up. Now, imagine that there is a string attached to the middle of your chest. Imagine the string is being pulled upward, exerting a bit of an upward force on your chest. You should find that naturally pulls your body into a confident posture.
Hopefully, imagining one of those two things will provide a natural way to take on good posture. Chose a common event to trigger you to imagine it, like every time you rise from a chair or every time you go through a door. Keep imagining it over several days. Eventually, it will become second nature.