Girls will sneak off with clothes to change into or simply wear their friends clothes after they leave your house - the issue isn't what you allow them to wear, but the values you instill in them in the first place.
Speaking as someone who admittedly is no expert on the subject of raising kids, I believe you have hit the nail on the head. I have my opinion on the subject and I'd like to get Staci's input on it, if she doesn't mind.
I think it comes down to discipline and boundaries. Not discipline as in punishment, but discipline as in understanding yourself and your place.
I believe that children, both consciously and subconsciously, want to know and define their place in the family, their place in school, in the community, and their place in life. The way that they find that place is by establishing their boundaries. They establish those boundaries by testing the limits of what is and is not allowed, and it starts as an infant. When they hit a wall, they mark that wall as a boundary. Once they have established enough boundaries, then they understand (or at least begin to understand) their place, where they belong, and what is expected of them.
When parents refuse to set boundaries for their children, whether it's at all or in regards to certain issues such as dress, the children will continue to test the limits until they reach a wall. When the parents don't set the boundary(ies) eventually someone else will, whether it's a youth minister, a teacher, a principal, or a police officer.
This is, I believe, why it becomes harder and harder to "instill values" in a child the older they get. It's easy to expand boundaries, but it can be extremely difficult to shrink them. And that's also, I believe, why it isn't merely about telling your child that things are either right or wrong. For some that might work but, for most, they aren't going to take you at your word and are going to test those boundaries. If those boundaries aren't there, even though you tell them something is wrong or dangerous, then they are going to continue to do it since that action clearly lies within their "place" in life.
What is most disturbing to me is that by neglecting or refusing to set boundaries for their children, parents are handicapping their children's future and setting them up to fail. Unless the child learns to "grow up" on their own, they are going to be smacked in the face with boundaries that they never knew existed as they get older. Whether it's, as I said, a teacher or a principal or an employer or a police officer at some point they are going to test a boundary with someone that they shouldn't test. And maybe mommy and daddy will be there to bail them out if they're still in high school or college. The sad thing is that while that helps prevent or lessens whatever consequences the child would have faced, it only further reinforces to the child that they don't have boundaries.
As this relates to dress, the all-too-common response is "that's not my fault" when possible consequences of improper dress are suggested. This all goes back to a lack of personal responsibility, which in turn goes back to a lack of boundaries. And it's a complete fallacy. While each person is completely responsible and at fault for their own actions, persons who antagonize those actions are also at fault.
If you stick your hand into a lion cage with a handful of raw hamburger meat you shouldn't be surprised when a lion bites off your hand. You also don't have a right to scream "unfair" and "why me" because a lion bit off your hand.
In the same way, if a girl dresses like a slut and a whore then she shouldn't be surprised when she gets treated like one. And, while the boy is completely at fault for his actions, she doesn't have a right to scream "unfair" and "why me" when a boy takes advantage of her.