How to subtly get rid of a friend?

There is NO subtle way to do this all at once. Either you ARE subtle, and you just find a way to hang out more with the other girls and not tell "Sarah" about it, and do that more and more often so you just aren't seeing as much of each other over a longer period of time, or you just face the issue head on, ask to talk to her in private, and explain that you don't feel you can hang out with her any more because you don't have as much in common as you used to. She IS going to be hurt and angry about it and you will just have to deal with that as politely as you can.