Fear has its good side: The perception of public speaking as difficult and demanding adds to a confident speaker's power, because people are perceived as more knowledgeable and confident simply because of their ability to conquer the dreaded task of public speaking. That confidence comes from within; once you believe you have the ability to be a confident speaker, it's a lot easier to be just that.
The best way to bolster your confidence before a speech or presentation is to think positively. Saturate your mind with positive thoughts. Repeat to yourself any positive catch phrase that appeals: "I am poised, prepared, persuasive, positive, and powerful. I also feel composed, confident, convincing, commanding, and compelling."