I really appreciate it when retail workers are nice to me (especially since often folks don't know what to make of my age/gender and so either talk down to me like I'm a 12-year-old boy - which I'm not - or act weird and awkward).  But I try to remember that everyone has crappy days, and not assume that because someone is grumpy one day, they're generally a bad person or not doing their job right.

come to think of it, that's probably a good approach to everyone, not just retail workers.