Big ideas come from the unconscious. This is true in art, in science, and in advertising. But your unconscious has to be well informed, or your idea will be irrelevant. Stuff your conscious mind with information, then unhook your rational thought process.
Every presentation has to start with an idea.
If you are a regular speaker who gets invitations to present at events, the event organizers might request that you speak on a particular topic to fill out the conference theme. But more often, especially when you first start presenting, you will be coming up with your own idea.
Your idea is the central linchpin of your presentation, and while it is possible to have a poor talk based on a good idea, it ...