Body language is just as important in communication as the words that leave your mouth.
Whether you're sitting or standing in front of your audience, work to improve your posture. Stand or sit up straight with your shoulders back, and keep your head held high.
This will make you appear bigger and more confident, and will help you feel more confident as well.
Plus, you'll get the added benefit of aligning your body so you can breathe--and therefore speak--more efficiently.
Posture can demand a lot of work, so make sure to practice in advance.
- Jayson Demers (modified by Amr Badran)