The oldest chestnut in public speaking advice is to “tell ‘em what you’re going to say, say it, and then tell ‘em what you said.” The idea is that repetition will hammer things home to your audience and help them remember.
Unfortunately, that’s bad advice today for a number of reasons. First of all, the only thing that your audience will get when you go at them with a hammer is a headache. Audiences these days are extremely sensitive to having their time wasted, and they’re easily distracted. So when you start with that agenda slide (tell ‘em what you’re going to say) their attentions immediately wander, they pick up their phones, and you’ve lost ‘em.
Instead, launch right in with a framing story or an idea that will grab their attention and at the same time tell them why they’re there. That’s what audiences want to have answered right away – not what you’re going to say, but why they’re there.
After that, the art of public speaking is the art of deciding what NOT to say. The urge, when you combine expertise, adrenaline and an audience, is to tell that audience everything you know. Unfortunately, long after the audience's enthusiasm has waned, because they're overloaded with information, you'll still be going strong — because you love the subject!
So you need to decide what the one vital idea is that you want to get across. And one more thing: your emotional attitude toward that idea.
A great presentation is composed of two things: one interesting idea and the speaker's emotional attitude toward that idea. It's that simple. Don't lard up your speech with caveats, asides, extras, nuances, added thoughts, one more thing, or anything else. Stick to your well-honed subject and make your attitude clear and your audience will love you.
Even more important, they'll understand you. And remember what you say.