I think a discrete continual reminder might be appropriate. Something like a sign or a slide reminding everyone to turn off or silence their phones during services.
Keep in mind while you are preaching - the guy who looks like he's "texting" might actually be looking up your sermon text and following you along more closely than anyone else. Or they could be posting your most quotable excerpts on AFF!
I'd also want to remember that maintaining a sense of sanctity and worship in the sanctuary is something you will always have to do; lots of gentle and even encouraging reminders all the time. Your announcement from last Sunday will have to be repeated - without you being cross for having to repeat it - time and time again.
In time, more and more people will be doing it for you so your burden can focus on the altar.
