Perhaps the very foundation of a strong sense of modesty is strong self-esteem. To stand up against the media, the majority, the norm? To say to a superficial society "I really don't care whether or not you think I'm 'pretty' based on your standards"? To treasure your virginity, treat your body as a temple? Now THAT takes a high level of self-esteem.
That's why I've decided to support the Campaign for Real Beauty. Click on the link to watch the commercial. It nearly brings me to tears. Maybe it makes me a sap, but I don't care. I look at the adorable, extraordinary girls in the ad and read the overlaid text about their individual body image issues, and I want to hold them close, tell them they ARE beautiful, they ARE everything, they DO mean the world, and all the other things I wish I'd let register in me when I was their age.
Not to sound trite, but we have to start with the young ones. Do you know a 9-year-old with freckles, a long braid, and low self-esteem? Then tell her that she's wonderful. That she's lovely and amazing. You can't say it enough. She can't hear it enough.