When someone has poor posture it is likely that their muscles in the front of the body are tight and the muscles in the back are not active. In order to use exercises to develop good posture, you must strengthen your ability to depress and retract your shoulder blades.
Regardless of what exercises you do, though, the best thing to do to reinforce good posture is to actually practice having good posture. When you sit down keep your shoulder blades pinched with your chest out and your torso should be tacked over your hips. When you're standing, you should be exaggerating good posture.
Refer to my YouTube channel for some exercises or contact me firstname.lastname@example.org