For our team the major way we get girls involved is through friends. Last year we had two girls (one of which graduated), but this year we had 5 or 6...(though they're all graduating - so I guess we have the same problem as you...). If one of their friends is involved and tells them what a great thing it is their more likely to listen to them than to read a poster hanging around the school or come to a meeting. Plus they'll know someone and feel more comfortable.
