Ask her questions about herself to help her open up. Make sure you listen when she wants to talk. Be patient with her. Compliments are always a win. Just don't be pushy especially if she's the quiet type.
First and foremost don't act like a creep. That's the first signal/vibe we're scanning for upon approach. Is this guy a scary creep? Is he going to rob me? Is he going to kidnap and rape me? Is he going to start stalking me? Is he going to come onto me? Will he go away if I say no? Should I run?
You need to make yourself approachable and friendly and not creepy. Timing is everything. Know when to back off and give her space. If we get the sense you're not the crazy obsessed type or going to try anything, the more likely we'll give you a chance. Act like a weirdo, we'll avoid you.