Lots of ways to do that.
My fave right now is making the girl get mad at you for not inviting you to something (works well for someone you've talked to before) or getting the girl interested in something you are going to do. From here you just say something along the lines of, "how can/could I invite you?" let her respond, most girls will say phone or facebook, some will say never mind-- if they do that just say that you feel bad and don't want them to feel left out, if they still are resisting you probably don't want to be with them anyway.
I like this because it creates a connection with the girl. You two have something to look forward to and that sells the now a lot better and she will be looking forward to your text
