Lose their attention.
Think about it, if you've lost people's attention, you've lost their interest. The purpose of public speaking is to move the conversation forward on a one-to-many basis. And without interest, the conversation cannot progress. Your presentation fails.
So how do you capture and retain your audience's interest? For starters, the 10 assignments in Toastmaster's "Competent Communicator" handbook will help you with many critical elements of this task. Techniques such as body language, vocal variety, research, and organization are powerful tools for engaging a group of people with your speech or presentation.
But because my formal background is in music, I would like to focus on the most auditory and musical aspect of these techniques: vocal variety. In music we would call this dynamics and/or expressiveness. I would also argue that it is one of the most effective speaking techniques for retaining an audience's attention.
Beginner: Avoid the Monotone
Think of a time when you've listened to someone drone on in a monotone; now think about what you remembered from them. Probably not much. This is because an unengaging or even off-putting vocal presentation creates a barrier between the audience and the speaker. It clouds the meaning of the speech.
You may be thinking, "I don't have a monotone delivery," which may be true. But the fact is, most of us have our comfortable range of expressiveness and tone of voice which, honestly, can be pretty repetitve at times. To make absolutely sure your speeches break out of your vocal comfort zone, you need to put on your "stage makeup."
Remember that you are now the center of attention, perhaps even in a fairly large room. This means you need to project. Put forth more energy, more charm, and more expressiveness than you might think is necessary. Imagine that there is a person way in the back of a 1,000-person hall, and try to reach them and move them with your speech. If people can't hear you, or if they have to struggle to make out what you're saying, they will tune you out.
You might think that turning up the volume in this way feels forced or inauthentic; but we're often the least objective judges of how we come across. Plus you'll never know if you don't try. Your Toastmasters group should be a place to experiment with this and any other techniques, and your evaluator will provide feedback, so don't worry about any of that. Just give it a try!
Intermediate: Match Your Dynamics With the Mood
You speak to convey an idea. That idea has a mood, depending on your audience, yourself, and how everyone relates to the idea. A masterful performer, whether in speaking or music, will have a vast range of dynamics available to help him or her make a point. If what you're saying is best said with a forceful tone, maybe bordering on a yell, you'll be hampered if the best you can do is sort of speak up a bit.
Likewise, if you're trying to lighten the mood with a joke but your voice is always matter of fact, the joke will fail. And how effective will you be at bringing the audience in closer by telling a secret, if your only volume level is that of a casual conversation? Imagine you're speaking to a group of non-English speakers and your only method for making your point is your tone of voice. That alone will help you broaden your spectrum of expressiveness.
Advanced: What Is Your Point?
In a way, this is the most basic component of improving your dynamic range and holding people's attention; but as is the case so often with the basics, it is also the most challenging. If you forget everything else and only focus on the purpose of your talk—the idea you want to convey and the effect you want to have on your audience—your vocal variety will take care of itself.
This is the essence of improvisation of all kinds: you follow the thread of your intention and the means of getting there surprise everyone, sometimes including yourself.
There are few things as satisfying as being so authentic to your purpose that the necessary vocal inflections, humor, and stories emerge almost of their own accord. You are the creator yet at the same time aren't self-conscious of creating anything—you just want your point to be heard. That's when the distinction between performer and audience, between speaker and the room, disappears. And there's nothing more attention-worthy than that.
What are your experiences with commanding a room? Share them in the comments below.