The next time you go in, just tell her you were so nervous the last time that you forgot your standard pickup line and ask her if she'd like to grab a bite with you when her shift ends...
"So tonight, when you're wondering what to say, or how you look, or whether or not she likes you, just remember, she is already out with you. That means she said yes when she could have said no. That means she made a plan when she could have just blown you off. So that means it is no longer your job to try to make her like you. It is your job not to mess it up."
-Hitch