Oh, that's such a difficult question. In Chile, at least, we would use USTED in the following situations:
1. The elder. I would use "usted" for anyone over 60years old (I'm 36) except, my mother, grandparents, in-laws, etc.
2. Strangers that I need to talk to circumstancially, for example; when running errands at a bank or in a government office, the bus driver, the subway clerk, etc.
3.To police officers or any of those.
4. When I was at school and in college, to talk to teachers/professors.
5. I also talk to doctors with usted, younger too.
6. I also use usted with any plumber, handyman or guy who's been hired to fix something in the house (yes, even younger to set up some distance with them)
Now, you need to take into account the following:
-some people don't use this "respectful" tone with the elder and just use TU for anyone.
-you may want to use it to set up some distance with people, for example, when they call me on my cell to offer me crap to buy, like services or loans or accounts or anything, I start calling them USTED rightaway (to hang up asap) and so that they don't feel they can "Tu-me" cause we seem to have similar ages or anything like that, so that sets up the tone and distance intended.
-people who clean facilities, always use USTED with everyone, older, younger, managers, assistants, all, except for other cleaning mates.
-Usted is occassionally used by some people in a sweet way to talk to their children (usually young), pet or significant other.
-I've seen on tv, that reporters treat anyone older as usted and ayone younger as tu. Presidents, senators, ministers, authority in general are all referred to as "usted" though.
But the general rule is, when in doubt; use usted.