i seem to have the opposite problem, well, actually no, i have mainly guy friends, but i do have some girls as friends and i get on with well, just it never seems to go further. only thing i'd say is stop worrying about it, and when you're talking to a girl just relax and dont think of things to say, if you do that you're not paying attention to what she's saying and the conversations going to go stale. Just talk, dont think of topics. just pay attention to what they are saying, don't be afraid to make jokes or laugh and just chill.
You just need to find the right type of girl. I am the type of girl that would rather hang out with a whole bunch of guys. I find a lot of girls are too uptight and need to get over themselves and lighten up a bit. I'm not saying all of us are like that it just happens sometimes. I have only find a handfull of girls that I can actually hang out with and have a conversation. It just seems that guys are a lot more relaxed and know how to have a good time!