If someone's texting in your audience, here are three very specific strategies you can try. For starters, simply walk over near the person, especially if they are in the front row. Once you physically get close, typically they're like, "Oh oh, I got busted." Without you saying anything, they'll put their phone away. Now, the key here is you're not looking at them. You're looking in another direction, so you don't seem like you're bothered or rude or anything like that. That's one strategy.
Another strategy is to see that the person is texting over here and reference them while looking over here. "Oh, as Jim was saying to me before the class..." And now Jim's like, he hears his name. He's looking up, so he is brought back in.
Something else you can do is ask the person a question, but you're not looking at them. You're far away, so you don't seem like you've pounced on them. "Jim, has that ever happened to you?" Now he's going to bring back that face up, the phone goes down.
But I urge you, you've got to nip this in the bud, because if you let your audience start talking, texting, checking e-mail, they're not going to respect you. There will be a ripple effect, and you will have lost them. So nip it in the bud.