Last time I went to a conference I was actually a little surprised by the number of people at the talks who where clearly working during the entire thing. Why bother, you most likely pay for the plane ticket, the hotel and the conference it self, and then not pay attention? If you're only interested in one talk, then just go to that one and then leave.
Given that people mostly show up to technical talks of their own free will, I don't think you need to trick them into paying attention. It's fair to assume that they are generally interested and focused on what you have to say.