Pretend that you didn't realize there were any other free seats and sit next to her. Strike up a strange compliment like for example 'This is going to sounds really wierd, but you have really nice arms' (taken from personal experience of what a girl said to me), but of course find something interesting about her that you could compliment on, then keep smiling at her, see what happens.
Guys smile too much anyway, which reduces the mystery and challenge girls get from finding out if you're happy or not. Next time you meet a pretty girl don't smile, i dare you



