To keep your audience alert and involved in your presentation, it can be very helpful to do the opposite of what they expect.
If they expect you to yell, whisper instead; if they expect you to speak, remain silent (while maintaining eye contact).
Of course, surprise is a staple of humor. Some of the most effective humor comes from surprising your audience. But the element of surprise can also be used for dramatic effect too.
Of course, this technique should not be overused. Surprising your listeners too frequently may annoy them instead of keeping them interested. So save your surprises for when they're truly needed and most effective.