Introduction to Multitasking
As a speaker you want to say don’t multitask. Stop doing email or engaging in other conversations while I’m talking. Pay attention to me. Multitasking does not work. It may not be diplomatic to say this, and even if you do some members of the audience will not believe you and will continue to do their other activity. They won’t learn what they should from you, but they can claim credit for being in your session.
This situation is really like many workplaces. Many job descriptions say “Must be able to multitask.” What they really are saying is “Must be able to be inefficient.”
For the most part people can only perform one complex task at the time. When we “multitask” we are simply switching from task to task. When we switch tasks the switching process per-se requires use of our processing capacity. Additionally while we are doing one task we cannot do much with the other task. So, if someone is talking on the phone while driving during the period of time they are talking on the phone they may miss critical incidents such as a child running in front of their car. This may result in a real tragedy. Likewise while attending to a complex highway condition they may miss important details on the phone call. This may not necessarily be tragic, but it can be embarrassing or result in a serious miscommunication.
The Activities
So, what can we do? We can introduce a couple of activities in our program to demonstrate that we really cannot multitask. Then we can say anyone who did not pass the multitasking test is not allowed to do email or engage in unrelated electronic conversations during this presentation. I cannot guarantee that this will work, but it should generate some smiles and perhaps making your program into an experience may have impact on your audience.

Multitasking A: Stand in one foot. Eyes closed. Recite alphabet backwards.
Demonstration #1: Stand on one foot
Ask everyone (except those with an ear infection, balance problem or weak legs) to stand up and close their eyes. Next ask everyone to lift their right foot off the ground. After a few more seconds ask everyone to recite the alphabet backwards loudly. Remind everyone to keeping eyes closed and their foot off the ground. Make a few photos or video. You will see people starting to sway and the recitations being less than perfectly even. When people cannot do this you can remind them that standing on one foot is simpler than driving (or listening to you) and reciting the alphabet is simpler than engaging in a conversation about a complex topic.
Demonstration #2: Draw a “6” and circles

Multitasking B: Draw clockwise circles with your foot and a big #6 with your hand.
Conclusion
Hopefully, people will pay more attention to your talk (and other talks which follow.) Maybe they will also think twice before using the phone while driving. Perhaps, lives will be saved.