Just want to second (third, forth?) the open communication, honesty and education. I will go now and "google" some books that can be useful and if something worthy comes up - I will post it.
I am another mama that got pregnant at 16, had my DS at 17 (and started sex at 15 - looking back at it I could have gotten pregnant right away).
My parents only started somewhat talking to me about sex when I was doing it already
It was more on the lines "You are not doing
it, right?" "Of course not!!!" (yeah, right) "Good". End of conversation. The best would have been if I knew the options available to me about birth control...
You are on the right track about being open about it. As other's said - you can not stop her, but can educate her. If she feels safe asking you for advice - you are miles ahead of my parents for example...