Preparing daughters for sex...

I am 23 years old. My god-daughters are 11 and 15 years old. It was not too long ago that I was in high school and I remember the blatant pressure to have sex at a very young age. These girls look to me for advice in much of their life issues. I know (or at least hope) that neither of them have had sex yet. I want to know the best way to either scare them into waiting or counsel them into waiting.
Does anyone have any life-experiences that they might share, or any advice on what I can tell these two girls to help them make the best sexual decisions in life???

Dawn80