Be honest. By that I mean let her know it is highly unlikely she is going to find her "soul-mate" as teenager or even in college. Our early dating experiences and relationships are an exploratory experience which helps us determine what we want and do not want in a relationship.
Take your time getting to know someone. In fact don't rush to be exclusive with anyone until you have truly gotten to know them. Keep your options open.
Don't let what is going on with your girlfriends effect your dating practices. Just because your friends are getting married, settled down, or having kids does not mean you have to turn your current relationship into "forever".
Avoid the trap of "competition" with other women to win the heart or attention of a guy. The more options he has the less likely he will want to be exclusive.
Don't define yourself by your relationship status! Stick to "your shopping list" when it comes to traits you want in a mate.
Don't fall in love with "potential". Find someone who (already is) what you want. It's a waste of time trying to "change" water into wine or fit square pegs into round holes.
Explain the natural "gender difference" between men and women. Sex and sexual attraction is a primary motivation for the majority of young men when it comes to approaching girls. He has not been programmed to think about being a husband or having children. Dating for him is all about having "fun" in the here and now. Not that there is anything wrong with that. Most people would be better off if they didn't get married until after they were 30 or had established a career path.
Don't confuse sex with love! And never have sex with a "hidden agenda". In other words don't think that by having sex it will make a man be more loyal or fall madly in love with you.
Never separate your mind from your heart when making relationship decisions. The purpose of the mind is to protect the heart. If something doesn't feel right it's probably NOT right. In order for him to be "the one" he must also see you as being "the one".
As cobrien stated (warn her about date rape, putting herself in harm's way, as well as educate her about "safe sex".) We'd like to believe our kids will not do the things we did or that we are closer to them than our parents were to us. However once juices start flowing, heart starts beating faster, and social/media pressure is applied "mistakes" are made. Hopefully your advice over the years has prepared her for the way the world works. There are no fairytales.