Create signage with the poster board and markers indicating that phones need to be turned off. Visual clues, such as a cell phone inside a red circle and bar across it, will alert and remind audience members that their phones and electronics must be turned off.
Place these signs where patrons will clearly see them before entering the presentation space.
Play the sound of a ringing cell phone over the speaker system just before the event is about to begin. A gimmick like this will remind the audience of their cell phones and prompt them to think about the status of their own phone.
Follow this gimmick with a gentle vocal reminder that says, "Please turn off all cellular phones and personal electronics for the duration of the event."