All day long we choose what we will listen to and what we will tune out. Understanding that, a speaker knows what he must do: capture the listener's attention, pulling it away from television, radio, Muzak, and that building custodian who's mowing the lawn in apparently endless circles. Even if they win that half of the battle, speakers still have to make listeners want to keep listening.
The key is variety. Like Sesame Street, we need to change the focus, the speed, the style, and the interaction level frequently. We know from the research done on learning styles that people learn in different ways (see yesterday's blog), so we have to consider that some in your audience will want to listen quietly, some will need to discuss, and some must move in order to process information. A speaker's job is more than just relaying information; otherwise we could read from prepared texts (ALWAYS a no-no!)
Understanding the dynamics of communication means that a good speaker will add specific elements to a presentation. I've seen authors (obviously well-taught by their speech teachers) who brought visual aids (Steve Hamilton does a bit with his book covers in foreign languages), created little "mini-mysteries" for the audience to solve, and one who even sang for us.) These are all things that make a speaker memorable (although I would have passed on the singing; it was ill-suited to the genre and occasion).
If an author simply reads from his/her book, most people will be polite as their minds wander. But add a bit of music, a bit of reader's theater, or a chance for the audience to contribute and you've got a more stimulating presentation. Stimulation brings interest, interest makes memory, and memory turns audience members into fans.
The substance of your communication should be well planned and thoughtfully considered as well. Once you've got their attention, what is it that (a) you want them to know and (b) they came here to hear? I'd say that (b) is more important to the planning of your presentation and (a) must be subtly inserted along the way. When I give talks on mysteries, for example, I focus on what the audience came to hear about, mysteries. Along the way I mention my historical novel, Macbeth's Niece, at least twice. Surely someone in that audience appreciates romance in eleventh century Scotland, too.
Back to your high school speech teacher. She was right: the key is preparation. When you write, you plan your path, you constantly check to be sure you're doing what you set out to do, and you put yourself in the reader's place to see if he's getting what you're saying. The same applies to speaking, so winging it in any sort of oral presentation is no more acceptable than unedited writing.