Simple question: What do you say, when you meet a beautful girl on the street and she smiles at you?

I'm sure many of us have been in this situation:

You're on the train, bus, shopping, whatever, a beautiful girl walks by and looks or smiles at you, you smile back, maybe say hi - or in the worst case: you look away, since you were somehow thinking about other stuff or maybe just too shy.

Suddenly you realize: this was the moment, where you should have said something real.

I guess this happens SO OFTEN and I'm unfortunately one of those guys, who doesn't do anything.

How are others doing? How do you approach (and keep the conversation going or maybe get her contact details for a next meeting)?

I guess the answer must be silly, but I didn't figure it out yet :)