One of the most successful "tools" I've used to keep students focused on PowerPoint presentations is rather abstract. I'll randomly insert images not relating to the subject material. The effect is similar to having the wrong slide in place. For example, I might have a dry history lecture interrupted by a photo of a llama. When the slide airs, I might yell, "LLAMA!" Although it get's a bit off subject for the moment, I've found students stay awake and pay attention. Students look forward to the little comedic breaks and tend to hyper-anticipate when they might arrive. In the end, students are more alert and stay focused on the presentation.