A husband’s opinion has a major effect on the clothing the wife chooses to wear. If he complements her on borderline tznius things (ultra long shaitels, skirts that are too fitted, a lot of make-up) than that is what she’s going to strive to wear. I think men need to pay attention to what they’re admiring and complementing (so too, peers/ friends, mothers).
It is sticky when a father comments on his daughters mode of dress (to notice that she isn’t dressed tznuah is to also notice her as a woman and not just his innocent child). I think if parents question their daughters mode of dress, it might be more effective to offer to take (or in the father’s case, send) her (with mom) on a shopping spree for new clothing (and n the process, guide) (obviously, this depend on finances).
The time to educate is when kids are young. I think telling a 16 year old that her mode of dress is unacceptable (when she attends a school where her peers are doing it or is exposed to it firsthand) is a recipe for rebellion. Teenagers are first recognizing their own beauty and begin to equate self worth with it. They don’t see where their choices will lead and how it can spiral out of control.