The most important thing is to practice:

Susan Cain, author of Quiet: The Power of Introverts in a World That Can’t Stop Talking, and an introvert herself, is now a professional public speaker.

How did she overcome public speaking fear?

She practiced in front of small, supportive groups to desensitize herself.

More notes about this article:

http://pandawhale.com/post/28776/how-to-overcome-fear-of-public-speaking-and-give-a-great-presentation