Cashiers, as someone earlier has said, don't really view customers as "people". They view them simply as part of their job and a requirement to deal with to get paid. To make matters worse, about 99% of customers interact with cashiers in a very monotonous, robotic way (i.e. "Hi, how are you? ... Thanks a lot!"). So the key to success with cashiers (or anyone in the service industry, really) is that you must first break out of that mold. This isn't really hard to do, considering that any interruption in the monotony of typical customer interactions will be viewed as fresh or different. You'll immediately pique her interest because of this (in most social interactions I think this is one of the more difficult aspects of picking up women, but it is the easiest in dealing with cashiers).
Say something funny or do something crazy/stupid to break out of that mold. Then, once you have her interested, it's just about building that interest like any other girl.
Say something funny or do something crazy/stupid to break out of that mold. Then, once you have her interested, it's just about building that interest like any other girl.