A friend forwarded an article from the Wall Street Journal titled Why Do We Let Girls Dress Like That?
Around here, we don't. We have many conversations, starting when they're young, as we walk down the street.
Me: Huh. What do you think that girl wants you to notice first about her?
Kid: Her butt [or legs or chest].
Me: Why would she want that? (conversation ensues, ending with...) So what do you want people to notice about you first?
I can't guarantee that it works, but it does open the door to talking about the fact that what you wear sends a message.