WesleyC - 9:42 pm on Mar 12, 2010 (gmt 0)
I've fought this battle myself--and it's all the more difficult for me, since I have Asperger's (though it's relatively mild).
The thing that helped me break through the fear was speaking during robotics competitions--we had to present our team as a company to a panel of judges, explaining why they should choose our "product". The judges were always friendly people, and enjoyed interacting with us. Once I realized that many or most of the people I was speaking to really wanted me to succeed, most of that fear went away.
Fast forward a few years. At this point, I've spoken during many of my college classes, taken a public speaking course, taught web development classes, and worked in capacities that regularly require me to make presentations, often to quite critical people.
The key for me was that early success. If at all possible, start by speaking to smaller groups that are likely to be supportive--it's a great confidence booster, and confidence is one of the things every great speaker has.
Another thing that vastly improved my speaking ability was the public speaking course I took. The professor I had was very tough, but fair. That course forced me to prepare every speech I made exhaustively--I had to know what I was talking about forwards, backwards, sideways, and upside-down in order to excel. If possible, check at a local college; many of them offer public speaking courses.
Reprint has some great advice, especially with respect to visual aids and knowing your audience. I would also suggest that any visual aids you use should be complementary to each other and to your speech--it may seem obvious, but you would be amazed at how many slideshows I've seen with outright incorrect or irrelevant information, random funny pictures, clipart dogs and cats, backgrounds that changed for every slide, and other visual clutter that really didn't relate to the presentation itself.
Another useful thing is to clean up localized accents, habitual phrases, and other verbal clutter, especially if speaking to audiences from other geographic areas. Coming from Kansas, I never really started with that much of an accent; but it took me months to realize why there was a half-confused look on everyone's face whenever I mentioned "pop"--apparently, most people call it soda. Looking back on those speeches, I know that these people were trying to process what I was trying to refer to, and that they probably completely missed the point of what I was saying in the rest of the sentence. In the vein of verbal clutter, I'm constantly reminded of one professor that would end nearly every sentence with the words "for us". One day I counted how many times he said those words during a class period and multiplied that by the length of time it took him to say it. The end result: over the course of my college career, I realized I had spent over 6 hours listening to him say "for us". Ouch.