I know this post is from a while ago, but thought I would add my two cents.
Since my child could talk, just over age one, I introduced the concept of "You are in charge of your own body." It came up because of situations like other kids pulling, hitting biting her etc. in daycare. I basically told her, that she is in charge of her body and if she doesn't like how someone is touching her or making her feel, then she should use her words to say "No! "Stop it! That hurts!"
Now, she is 4 years old. We have continued the above discussion, including the idea that she CAN say "No!" to adults and if she is ever not sure about how she is feeling she should talk to us - we will never get mad for her saying no if she is feeling bad or worried.
We are also now at an age where she has learned the distinction about lies and pretend and truth. So, now we talk about how some people will lie to kids and that she needs to follow the rules of mom and dad (like, don't leave the playground) no matter what someone else says. When she was around 2 1/2 to 3, I started casually using scenarios to see what kind of decisions she would make - like what if another daddy at the playground wanted you to help him find his lost puppy? would you go help him? The answer is no, I would come tell mommy and you could help him.
Now, at such a young age, I dont't trust her at all to actually do any of this, so I naturally never take my eyes off of her. But I want to encourage her to think independently and not be a people pleaser who will just do whatever any adult tells her,
I have also taught her recently, if she loses mommy while we are out, to stand still and not leave. I have played and practiced pulling her arm and asked her to yell NO and pull away - as if someone else where to grab her what would she do, etc.
All together, this probebly sounds sort of severe, but I basically have done all there things gradually over three+ years and look for opportunities to introduce them in a relaxed and matter-of-fact matter. I never sat down with her for any one "talk."
Recently, she went up to a daddy at a playground and asked him to push her on the swings while I was busy with our baby. It scared the hell out of me. This was the first time I actually said, "some people want to do bad things and just because he is a daddy doesn't mean he is a nice man, although he probably is ... blah, blah...
Mostly, I want her to use her instincts = "trust her tummy" Oprah has talked about this several times in talking about victimization. So I also take opportunities to talk about this, for example, there is a Betrix Potter story my daughter just started reading (Jemima Puddle-duck). In it, the duck meets a fox who wants to eat her for dinner. The duck is cluelsss, but in one conversation is surprised when the fox is suddenly not too nice to her. I use that to point out to my daughter that the duck should have listened to her feelings of discomfort and run away.
For me, I worry a little less about the creepy guy in a car who might grab a child and much more about the dad down the block who is an abuser. That is why I want my daughter to know she can stand up for herself AND I won't be angry if she does. She is a nice kid, so I know if she ever says something "disrespectful" to an adult, it is because there is a good reason. I know it is scary stuff, but it is scarier to believe your child won't know the right thing to do.