HBS Working Knowledge writes about how to prepare for a speech:
Phillip Khan-Pami, a coach and competition-winning speaker himself. Khan-Pami suggests beginning by identifying your core message: “When you have completed your presentation or speech, what will people remember? What will they take away with them, to apply and change their ways, and one day perhaps even thank you for? What one sentence will correctly sum up your entire presentation? That’s your Core Message…Make no mistake: Your listeners will take away a core message of sorts. They will carry away an impression of what you were saying. It may be complimentary, it may be less so. It may be about you and your delivery, or it may be about your content. If you want them to receive and carry away the right messageyour Core Messageyou must first identify what you believe it to be and write it down.”
Once you’ve developed that one-sentence summary of what you want to say, you’re ready to brainstorm supporting ideas, arguments, anecdotes, and information. Then, Khan-Pami advises, test everything you’ve come up with against the core message. Ruthlessly eliminate anything that doesn’t support your message.
The article provides a number of formats to structure a speech: