Aside from the content, the best bit of advice I have ever had regarding public speaking is speed.
When you are talking to people you know in a small enviroment you will be speaking fairly quickly.
When you speak to a large crowd you should slow your speaking down. In your own mind reduce how quickly you speak each word, to you it should seem slow, but to others it will appear normal. If you are nervous, it is apparent to others by how quickly you rattle your speech/presentation out. Slowing it down will make you appear more confident, which in turn will make you feel more confident.
good luck