I used to have the same problem, although I could score once in awhile, but I went through most of high school and my early 20's with not getting too much.
Then I moved away from my home town and my friends and found out what the problem was....it wasn't me at all, it was my friends that were scaring the girls away(read earlier posts on pg2/3).
I should have picked this up along time ago because all the time I did hook up with a girl, none of my friends were with me, I was usually out of town or they had ditched me to go and do something stupid...as soon as they left chicks would talk to me and I could usually pick up with out too much trouble, I always put it down to dumb luck.
But when I was 21 I left my home town(Temora, pop 4600, 400km's west off Sydney) to move to Sydney to start a new job and I was living by myself and none of my friends were about I was able to pick up a girl nearly every week, it was crazy at one point my current GF was fighting with other girls over me it was damn crazy.
I figured it out when I took my GF to visit my family and friends and her disgusted response to my friends (it went something like this "ugh good god, you hung out with losers like these")gave it away.
Now I doubt that I would have any problem replacing my GF should we break up, when I go to a pub or club I just either go alone or when I see a girl that looks like she has the goods I just ditch my friends before she see's them.
Strange but true