I am not too active in the public speaking business anymore (not that I ever charged for speaking, either ;-).
A few years ago, I would do it about once a month, now it's about twice a year. But I have done it some 50 times, with crowds of up to 1000, and I think I am pretty good at it (but getting worse by the minute). And I have been to a million or two.
And for some reason, I have decided I want to talk about it. Many of the KDE guys & gals have to do it every once in a while, and a bunch will do it in aKademy so, maybe this can help someone :-)
Of course, this is surely the wrong way to do things for 90% of the people, so read without any care ;-)
Entertaining is better than boring
Obvious, right? Well, then why are 99% of these things so dull?
The main reason is, of course, that the one speaking is scared (not nervous. Scared.) , or that he really is a dull person on first contact.
Most of the free software people I know are not dull at all. But when you first talk to them, they are about as interesting as a brick.
That's probably shyness. Most extroverts don't make a living by staying immovile in small places and looking at the same three objects for hours at a time.
So, how can a shy person be interesting quickly? By not trying. For Christ's sake, don't make a joke (unless it's really unavoidable ). Just try to chill out, and speak about what you want to say, in a natural tone. Stammer if you have to. Think before answering questions.
Specially, allow the audience to ask questions during your exposition, if you can do it. Who knows, it may not let you say what you wanted to say, but it will let the audience learn about what they want to learn, which is just as good.
It helps a lot if you talk about something you really know. Don't try to overreach. I couldn't speak about C++ development because I am not good enough. Sure, I could give a crappy lecture. Just not a good one.
So, if you have to, stay on a simple topic. The audience will lead you from there, but if they lead you somewhere you don't know... well, say you don't, and then you both can try to figure it out or ask someone else.
Avoid slides unless absolutely necessary
One problem is that they will believe the slides are the lecture. They aren't.
If what you have to say can be said in 30 pages with 6 lines each, then why the hell are you taking 45 minutes to say it? If it can't, then what are they for?
They lock you in a path. Unless it's absolutely necessary that you get a specific message out, I say don't bother.
Use graphics? Sure. Use a live app showing how to do something cool? Sure. A blue background with a picture of app A saying "app A can do something cool"? Yeech.
To connect it with the other one, some people think slides will make things more interesting. Guys slides are incredibly boring. They are more boring than the speaker making funny shadows on the screen.
What the heck, put some nice screensaver hacks and leave it at that.
 I was once showing how to rip MP3s in konqi and asked anyone for a audio CD. I got one from a 16 year old. Written in huge letters on top of the CD was "crappy music". I won't say what I said, but you can say your own in the comments ;-)