I completely relate to this. It's very likely this happens because you're just too "friendly" with the women you want to be more than friends with. At least that was my problem.

I think some of us guys don't instinctively know how to escalate sexuality in our interactions with women, so we end up just acting like a buddy, and then that's all we become. "Nice guys" always lament the fact that women only want "bad boys." Well, the truth is they don't really want bad boys; they want someone who is confident and overtly sexual. These "bad boys" tend to be self centered and have an over-inflated ego which makes them believe themselves to be cooler than they are and god's gift to women... and they act like that... which comes across to many women (especially younger ones) as being confident and overtly sexual, so they are attracted to it.

NOTE: by "overtly sexual" I don't mean just trying to get in her pants or groping her all the time. That's just gross and annoying. I mean presenting yourself as a sexually capable being.

I never dated anyone in high school. I went through college dating a couple people, and then got married all without knowing this. I accepted every woman who seemed to want me because I didn't know how to attract the women I wanted. It was only after I was divorced and thrown back out into the dating scene, a little older and wiser, that I actually started realizing how to attract women.

Women don't have this issue because a large number of men are sexually attracted to them simply because they are female. This presents its own problems, so not to say women have it easy! But as men, women are not just sexually attracted to us by default unless we are just the most handsome guy in town. You have to present yourself to her as someone who is sexual and confident.

The best way to do this is simply by believing it yourself. I realize that this is very difficult, but you might try changing your appearance, buying some new clothes or something so that you feel sexually attractive and are confident in yourself, and that will come across. Make sure she knows you are interested in her as a woman, not just as a buddy. That doesn't mean confess your love or something, that's just creepy. I mean through your actions, compliment her appearance, touch her hand or arm as you talk to her, ask her out to do things that are unmistakably dates... I know this is difficult because you open yourself up to rejection, but that comes back to the confidence part.

Also, practice makes perfect.

I know it's difficult but if I can figure out how to get out of the friend zone, anyone can :-)