Just strike up a conversation...like "stuck working today, huh?" or make a joke about something you're buying. What the other answerers said...just be flirty and go with the flow!
Compliment it up. How often do people get compliments from customers at work?
flirt with her and make her laugh ..
Buy booze, small talk about a party you're having while you're "trying to find the money", then invite her to said party. If you're a total stranger to her, then I got nothing, sorry.
Ha sometimes the cashier will flirt with you too, I do that often and just forget where I am =p but just go with flow, and be yourself that's mostly how it works.