While it must be a big idea, you need to be able to communicate it in fifteen words or less.
Ryan Holiday argues that this is true for book pitching as well. If you want to sell a book (either for publishing or for the final consumer) you gotta put the greatest idea in one sentence. This is his exact framework.
Know yourself.
Great talk on the subject of coaching yourself for public speaking: video here
Know the difference between a good talk and a bad talk
I believe that the single greatest factor for a decent talk is doing your homework. And the greatest difference between a decent talk and a great talk is the speaker's experience on stage.
If you’re walking into a speaking gig without knowing your audience, you’re bound to fall flat and end up looking at the tops of their heads as they check their cell phones.
This is what Pedro Salomão calls "the speaker's fault". He has a great point on millenials: if they are bored, either in a lecture or a classroom, it's the speaker's fault. No one is bored while watching the world cup or an amazing keynote.
Know how to pitch.
A great framework on pichting comes from Dan Pink on this short video.