This may seem like a no brainer, but if you want good customer service, treat the clerk kindly. Don’t insult her. Don’t talk down to her. Don’t blame her for something that isn’t her fault. (Feel free to change the gender pronouns as appropriate.)
Basically, treat her as you would like to be treated.
I think everybody should work a customer service job for at least a year so that they develop some empathy and compassion.
The person behind the counter is a person, not your personal slave.
She didn’t make the rules, so yelling at her isn’t going to change them. It isn’t fair to her to attack her over something she has no control over. She feels just as frustrated as you do. Perhaps more so.
She isn’t allowed to defend herself either, so you just end up showing how much of a jerk you are if you attack her verbally. It isn’t an equal relationship.
She isn’t your friend. She has to be friendly to you. That is part of her job. If she isn’t friendly, she’ll get reprimanded. If she is your friend, she’ll give you her number or email address. Otherwise, don’t assume.
Don’t ask her out. Especially if she is married. If the only reason you know her name is because you read it on her name tag, don’t ask her out. Really. At least get to know her as a person first. Surely you aren’t asking her out just because she is female, right?
Don’t tell her your personal stories if they have nothing to do with what she is expected to do as part of her job. – unless she is actually your friend.
Don’t ask her to break the rules for you because you think you are her friend. She can get fired for breaking the rules. A real friend wouldn’t ask.