Remember to breathe. This sounds simple, but when people are nervous, they often hold their breath, or breathe too quickly. Take a deep breath, then exhale slowly. Practice this a few times.
Stand up straight. Keep your feet about a shoulder's width apart, and place a supportive hand on your stomach. Visualize good energy coming in with each breath, and negative energy going out with each exhaled breath. If you cannot stand, work on sitting as straight as possible, and complete the same exercise.
Relax shoulders and arms, and imagine stress leaving your body through the tips of your fingers. Gently stretch and loosen muscles, and shake the nervous energy from your body.
Visualize yourself giving a riveting speech, and capturing the audience's attention. Practice reciting the speech a few times, and get used to hearing the sound of your voice. Picture yourself speaking slowly and clearly from start to finish.