Speak out!

Unquestionably, the best way to learn how to speak in public is to speak in public.

Perhaps you're still in a position in which opportunities for speaking out in public often arise. But, if not, there are ways to create your own opportunities:

Ask a question at a conference and whenever a speaker asks for a volunteer to role play or otherwise assist in the presentation, do it.
Call into a talk show. This even gives you chance to practice your delivery fairly anonymously. (Do it on an NPR program, where the discussion is more civil. You can also check the program archives later to hear yourself.)
Volunteer to give a reading at your place of worship. (In my own case that was where I discovered how nervousness could be an asset instead of a liability.)

For tips on how to do this effectively, and even enjoy it, see the "Primer" at left.

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P.S. When I talk about "speaking in public," I am not referring to cell phone conversations.