Assuming that I had to do this, I would try it from a variety of angles.
First, since we are talking about young Christians, I would want to appeal to religious values. I would talk about how important it is to live up to what God wants us to be. I would talk about how it is hard, but that it is important to live a life that can serve as a witness to other people.
Second, on a more secular note, I would talk about the difficulties that can be encountered if one has premarital sex. I would talk about the problem of potential jealousy on the part of your spouse when you do marry. The more of a sexual "past" you have, the more likely your spouse is to feel threatened and jealous.
Third, I would talk about the idea that sex would be better within a committed relationship than outside of one. I would talk about how personal sex is and how vulnerable you have to make yourself to the other person. I would use this as a way of saying that premarital sex would not be as good as you would want it to be anyway.
Of course, none of these ideas is new and there is no way to know how well these arguments would work.