Fistful: in those situations just be honest or play it cool. I prefer the play it cool route because that will often lead to a moment where you both don't remember each other, causing a few laughs.
You: Heyyyyyy, It's been a while. When was the last time we saw each other?
Her: blahblahblah
You: cool, we should hang out some time. Got a facebook?
And you can get a potential date out of it too.
I mean, she smiled at you man. Did you smile back? It's not rocket science. It's just natural to smile back when someone smile at you, right?