What advice should you give someone who is about to a give an impromptu speech?

Just be yourself - Don't talk too fast.

Because the speech in impromptu the audience will not expect you to be "slick" so just speak from experience and ensure you get the audience to warm to you with a subtle joke (if appropriate). Introduce yourself giving a little background which demonstrates your expertise and experience in your subject.

The audience want's you to deliver interesting, meaningful information so before you go on stage make a few bullet points which will remind you of the important "take away" information that they can use.

Mick