Your posture is the first thing you say about yourself in a speech. Are you trustworthy, confident, intelligent? Correct stance will go a long way toward communicating these qualities.
What good posture is:
What good posture is not:
How to make good posture a habit in your speech. Practice you speech two ways. Give your speech in a mirror, paying special attention to your posture. Then give your speech lying flat on the ground. This will both help you to straighten out your back and to breath deeply, utilizing your diaphragm. When you go return to standing, try to replicate your posture and breathing on the ground.
Exercises for good posture.
Shoulder Circles: Stand with your feet about shoulder-width apart. Swing your arms backwards in a circle. Repeat 30 times.
Back Extension: Lie down on an elevated platform (chairs end-to-end, a bed, etc) with your chest and head over the edge. Place your hands behind your head, then raise your upper body using your back muscles. Repeat slowly 10 times.
Stretches: Stand with your feet about shoulder-width apart. Interlock your fingers in front of you. Push out with your palms facing away from you, and you should feel the stretch across the top of your back. Slowly lift up until your hands are above your head, then go back as far as possible.