You: "Excuse me..."
She: "??"
You: "I think you dropped something back there...."
She: Usually will swing back the way she came, then not seeing anything she dropped will look back to you...
You: "I think you dropped your smile...."
She: 99% of the time will smile...
You (Wearing a big grin): "Ah! I see you found it! You should wear it more often, it looks good on you!"
It is kinda cheesy, but even so, it is cute, complimentary, and she will have a warm fuzzy feeling about you. Now just be yourself and talk to her with confidence!
Good Luck!