When do you use "usted" in Spanish?

it is not a matter of respect alone but of respect AND familiarity. god and your mom and dad and grandparents too are usually TU, not USTED, because they are so familiar. so, in many cases, as you become more familiar to someone, you switch. but this is not simple, and there is no one rule. it depends in part where you come from, and what your age, gender, and socialeconomic status is. i know a young mexican who works in a store i often go to and he calls me TU, but his boss in the same store always uses USTED, though the boss knows me for a longer time and much better. they do come from different parts of mexico, though, and though each is of native american origin, each comes from a different native american tribe. also the boss is older and probably more educated. and it may be just personal taste. i don't know--and i don't ask. i just go with the flow.