This is a classic situation for the word "Poptart". I will warn you that it has never worked for me for girls that turn out to be GF material. But ho's always bite. It's fool proof. Next time she walks by just yell out "Hay poptart!" (you can also use cupcake). Immediatly after you say it she will look at you confused (not sure if you are talking to her). Then you gota say something like "what up" Then she will come over and start to talk to you. This is to find out if (poptart or your word of choice) was a insult or a compliment. The key is to use a word that can work for both. This will give her something to start talking about "WHICH IS KEY" . Now its up to you. Remember girls are really easy to get all you have to do is talk to them and your in. This is just somethiing that can break the Ice.
OH The other side of the line is, if she blows you off you can make it look like you were insulting her anywhay . so you dont have to hear lip on how you got dogged from your friends.
** Oh yea I have only tried it at a bar**
But seriously go up to her and talk there is a good chance that she wants to talk to. Also some girls get intrested in you just because you went up to them to talk.