Study other people who exhibit poise, and pattern your physical behavior on theirs. For instance, on a news talk show, pick someone who exudes calm and confidence. Look at that person's posture, listen to her tone of voice, watch the mannerisms and facial expressions. The more you observe, the more you can try it out in the privacy of your living room. You'll find yourself sitting up straighter, and you may even practice projecting your voice with poise, even though--or especially because--no one is there to hear you.
Take a dance class, which is a great way to develop balance and physical poise. Ballet will work as well as ballroom or jazz dancing. Gymnastics also provide excellent physical practice.
Develop your internal calm--it's as important as your physical demeanor. Study meditation or a spiritual form of yoga. Learn tai chi or chi gong, Chinese exercises that slow you down and help you grow strong and poised, inside and out.
Take a debating class or join a group such as Toastmasters. You'll learn poise in communicating, an area in which many people tend to get nervous and rattled. You'll find it easier to express yourself and remain in control even when others disagree with you.
Take an etiquette class, which could also be called a poise class. You'll learn manners, social skills, professionalism, and everything from dining to dating etiquette.