Speak. Stop. Start Speaking Again.

Early on, while working in radio stations, I learned that 'dead air' is a bad thing.
Dead air means silence, unintentional silence, that is. So, if I happened to be standing in the hall, for instance, and heard no music or voice for more than a couple of seconds, I would quickly check to see what had happened in the announcer's booth or the news booth.
Speakers and presenters, too, often think of silence as a bad thing. But, they should not. In fact, silence, as in a long pause, can be wonderfully powerful.

Pause for a moment before you start speaking, and you'll almost immediately have the attention and respect of everyone in the audience. Any whispering that had gone on will stop, as will the shuffling of feet and papers, and the opening and closing of briefcases and purses.
The same holds if you lose the attention of the audience part way through your speech or presentation. Pause, look systematically around the room at everyone in the audience, and you'll have them back with you again.

Pause for a long moment if you want to emphasize a point. When you pause, you not only get the attention of the audience, but you create a contrast between the silence and the sound of your voice.
You'll also find pauses helpful when you change from one subject to another within your presentation. In this case, the pause signals that something's about to change, especially if you foreshadowed the new subject as you wrapped up the preceding section.

Of course, you can also pause when you lose track of where you are in your presentation. Deliberately stop, look at the audience as if you had planned to stop at this point, collect your thoughts, and then start again.
In summary, don't be afraid of pauses or long moments of silence in a presentation or speech. They can get and hold attention better than almost anything you can say.