How do people retain a proper posture when when sitting, standing, and walking?

Here I am referring to a posture that is not overly stiff (like with high, tense, shoulders), but also not slouching.

What techniques can be used to

1. find the right posture for oneself?
2. keep up the right posture?

How do people retain a proper posture when when sitting, standing, and walking?

Here I am referring to a posture that is not overly stiff (like with high, tense, shoulders), but also not slouching.

What techniques can be used to

1. find the right posture for oneself?
2. keep up the right posture?

Additional update: does proper breathing have anything to do with keeping up a good posture, and if so, how does that work?