It is said that 10 minutes of meaningful exercise (See Benefit # 42 of the ‘Benefits of Exercise’ Series) is enough to train posture; I don’t know about that. But I do often recommend two of the exercises that I have seen offered to correct posture. One I recommend usually for an entirely different reason, but posture is complicated so lots of things impact it. The exercise that I am referring to here is Ball Circles with the Gym Ball (see ‘the miracle cure’ on this Blog for a series of Posts dealing with the Ball exercises.
The other exercise guaranteed to improve posture is Tai Chi. Recognized as a ‘posture-aware’ exercise, it’s benefits, along with improved alignment, include improved mindfulness and body control: this is why recent commenters are focusing on and referring to ‘strong’ Posture. Strength is a big part of it. Tai chi (the Perfect Exercise) trains that in spades!
Improve balance, alignment, and motion by doing these two activities, in particular.