How can someone learn to speak publicly--or give compelling presentations--with poise and and confidence?

Answer 1

Two basic ingredients to delivering outstanding speeches and presentations: what you say and how you say it.

What you say, the content of your speech or presentation, needs to cover all relevant topics for your targeted audience. Not only that, expressing your content using sentences that vary in length and construction with strong, active verbs makes your listeners more aware of and interested in what you are saying.

How you deliver the speech or presentation influences its overall effectiveness as much as your content. Perhaps more. Your facial expressions, voice intonation, body movements--in each section or part can bring about powerful effects in your audience.

Answer 2

Just like anything else practice practice practice! I agree with John above and would add that you may want to find a a few speakers giving speeches you found captivating and emulate that style, until you are able to find your own delivery technique.