Simon Cozzens wrote a good entry On Talking, which I might call On Speaking.

Confidence x Knowledge x Practical Application x Emotional Appeal = Interest

And...

All through high school I was quite lacking in confidence. I gave my first set of talks at OSCon 2000, where I was booked for two tutorials, two papers and three talks - starting very much in the deep end.

But I managed to get away with it in part because my confidence was provided by knowledge; I knew everything about Perl's treatment of Unicode...

And...

Practically, I like to position myself about a meter away from the script while I'm talking, so that I'm forced to work for memory as much as possible.