C.C.
I think it's good to talk about the differences between strangers and people we know - but the thing is, they've done all these studies with kids and if the person looks ok, the kid will trust them every time (or if they have a puppy, or a kitten, etc). Half the time kids are kidnapped by people they know, too. So I'm not sure I trust my kids to really and truly know the difference. What I tell my youngest one (she's 2.5) is that if she strays out of my sight for one second, we are putting the leash on her. We have one of those baby leashes/harnesses, and she hates it, but it's better to have her hate it than to have her kidnapped. All I have to tell her is, "Do I need to take the leash out?" and she will straighten right up and stay by my side.
With my older daughter, whenever we go to a crowded place, we go over what to do if she looks up and can't see us. I learned this from a friend of mine and it made such good sense to me - instead of looking for a policeman or a mommy with a stroller or whatever, we tell her to SIT DOWN the second she realizes she can't see us. If anybody asks her where mommy and daddy are, she says, "They will be right back for me." This serves 2 purposes - one, she won't go off with any random strangers, and two, I have to be constantly alert so I WILL be right back to get her if she is lost!