Word to call a person that works in a store

I seem to always have a trouble with this one; what do you call a person that works in a store? A clerk? A sales person? Neither of these sound right. Saying "person that works in the store" doesn't work that well either.

Is there a general single word that I can use that represents "a person that works in a store".

When you can't apply any of the fine terms provided here so far:

employee will always work.

In the UK we have shop assistant but there are more specific terms like checkout girl and the possibly pejorative shelf-stacker, as well as the general shopworker, retail worker and so on. The best word to use probably depends on what your person actually does.