I'm going to be serious.. unlike some of the other guys. (ah heck off!)
If you have a chance to talk to her, do it. If you pass it up too many times, she'll think you're just another guy riding the bus and think you aren't interested.
If she is holding a conversation with you, you're almost there. If she seems interested, at least somewhat, and is talking back, then you're even closer. Don't look down. Look into her eyes - especially when you first talk to her. You'll know what I mean when this happens, unless you lose your cool and get all nervous. You don't want to talk too much, because there's a line - once you pass it, you're just friends, and sometimes you can get stuck there. Not good.
Even if you can't talk to her much, just nod and ask her what's up. Blah blah, some small talk to make it seem like your bored and want somebody to talk to. From there it's up to you..