Spend your day getting amped up to talk to this girl.
Think about about you're going to say. Talk to other girls as a warm up. Think positive thoughts.
As for a general script:
Make eye contact. Smile at her (Watch this for info about the right facial expression
http://www.youtube.com/watch?v=tm_WPB4Qrww )
YOU: "Hey, I'm [Name]. I think we met the other day."
HB: "Oh yeah, you look familiar! I'm [HB]."
(for the sake of flow, [HB]=Amy)
YOU: "Amy, huh? Do you have a nickname Amy?"
HB: "No, I just go by Amy."
YOU: "Well my nickname is Mr. Poo-Pee-Pants. I'm sure you can guess why!"
[laughter]
YOU: "Hey, I was just passing by so I gotta get out of here. You seem like a cool girl. Give me your phone number."