i'm having a bit of the same problem that you're having. i'm part of my university's rowing team, as well as the rowing club (i happen to go to school in the same town i live in, so during the uni season i train out of the same club, but i race under my school's name), and i'm involved with the engineering community at school. the problem is that i know most of the girls in engineering (not too hard to do, there aren't all that many of them) and very few of them actually interest me, and the ones that do are usually taken. through rowing i've met one girl i like, but that situation's kind of up in the air right now, i have no idea if anything will come of it.
so i'm in two good sized social circles, and i've met one girl i liked. if you're going to university/college, classes can be a good way to meet new people. if you can get into a study group, or if your courses have tutorials or labs, then you have a great opportunity to get to know other students. if you're like me and you don't have a lot of members of the opposite sex in your classes, then you can try taking a course in another department or faculty that might interest you. this year i'm taking a first year psychology course because there's a second year course that interests me and this is a prerequisite course for it. this semester i've discovered just how much easier it is to meet girls in arts classes. my problem this semester is that i didn't really attend class or the tutorial very often so i didn't have the chance to talk to them very often, but it's definitely a good way to do it.
other than that, i'm with alex, if you see a girl you find attractive, go up and try to strike up a conversation. if it's hard at first just remember that you'll get better the more you do it.
that's my advice for the day, now it's time to stop procrastinating and start studying for my exams next week
kill me now