When I was younger my parents were always nagging me to standup straight. they wanted me to develop good habits and posture was one of them.
It is important to have good posture when you are speaking in front of an audience. When speaking you should stand up straight, shoulders back, chest out, stomach in, with your arms to your sides (unless gesturing). Good posture shows self-confidence. You will look better and more professional when you are speaking. Good posture provides a psychological effect on your message to the audience. When your shoulders are hunched over you appear less confident and sloppy or frumpy which the audience can look upon as unprofessional.
Don’t lean-to one side or sway back and forth this also can detract for you message.
When keeping your hands to your side resist the temptation of putting your hands in your pockets. Hands in pockets will look like you have bad posture and if you happen to have something in your pockets such as keys or change quite often without even realizing it your nervousness might show by playing with the keys or change making distracting noises. I have seen many speakers make this mistake. I suggest taking everything out of your pockets before you speak so that even if you put your hands in your pockets you wont shuffle anything.
It is important to note that good posture doesn’t mean you should stand so straight and still that you look stiff on the podium. Standing so rigid while speaking can also have a negative affect on the audience. The audience will interpret that as stage fright. when the audience perceives you as nervous they interpret part of that as you not having faith in your subject. Working on your posture while you are in front of an audience is not a good idea. constantly practice good posture so that when you are in front of an audience it will appear natural and you will make your parents proud.
George Gilbert's Motivational Humor 