I've approached these conversations less along the lines of 'strangers' and more a discussion of 'good grown-ups/bad grown-ups' and their behavior.
For example, a good grown up would not ask you to walk away with them. Good grown-ups would ask you where your mom and dad is and come talk to one of us. Only bad grown-ups think it's okay to invite you away from your family or the park, etc.
If good grown-ups lose a pet, they won't ask kids to help them find it. They could ask if you might have seen their animal, but they won't ask you to help them look. They know that is a grown-up job, not a kid job. Bad grown-ups will ask you to help them, but they know they're not supposed to. You never go with someone who wants to find their animal. Come and tell me if someone is asking you to do that.
When he was younger and I'd go to take a shower, we'd talk about how good adults DON'T want you to open the door for them without mom or dad. They know only mom or dad should open the door. Mom and Dad have keys, so we don't need you to open the door for us. Only bad adults ask kids to open the door.
Etc etc.
We review plans for 'what if you get lost', have my phone number and our address memorized, etc. During field trips I would put a card in his pocket with all of our information on it. Since we have a lot of mentally ill people who use our public libraries, the buses and the downtown area, I keep him close. He actually has shown a lot of discernment in who he will and won't initiate a conversation with. Due to that 'mentally ill' factor, I also have him use the family bathroom in the children's library or come in with me if we are out and about. He also knows about private parts, etc.
I try to let him know that the world is a pretty good place, but that there are some people who just aren't safe. He is at an age where he does understand a very innocent version of 'good guys/bad guys' and frankly, I do NOT want him to be afraid of strangers, but to be watchful for *behaviors*, hence the descriptions. He also knows that if someone grabs him to scream, wiggle with his whole body, to hit and bite if need be. He IS allowed to throw a tantrum and scream YOU ARE NOT MY DAD/MOM! LET ME GO!
Ultimately, my job is to inform him at an age-appropriate level and to ensure that he doesn't have more freedom than he can manage on his own. He does get quite a bit of freedom close to home, and I'm very aware of the fact that most molestations/abductions happen with known persons, not strangers. This is why I am careful not to make him afraid to ask for help-- "if you see a mom or dad with little kids, they are safe to ask". Chances are, a parent with kids of their own is NOT going to abduct my son, right? They already know how much work a kid is! ;)