Not to pat myself on the back or anything, because like almost everyone else I've had my deer-in-the-headlights moments in front of a crowd, but overall I think I'm a pretty good speech writer and public speaker. Whether it's a best man speech, eulogy, or powerpoint presentation, I've had positive reaction to the speeches I've given. It's funny because when talking without a script I tend to sometimes stumble over my words, but when I'm prepared with talking points for an interview or have something written which I've memorized or read, I feel confident and it comes out pretty well.
Confidence of course is a key element for an effective speech. Preparation is equally important. Add some theatrical training -- control of your voice and body language, basic knowledge about timing and rhetoric -- and you'll have any audience in the palm of your hand.
Listen to some of the best speeches and you'll see what works and what doesn't. The words are the first and most important part, but the delivery is the make-or-break factor. Writing a good speech is half the battle. The other half is presenting that speech in an effective manner.