I have been learning Spanish in college and the way I was taught is that we are to use "usted" with those that are professionals such as doctors or teachers, and also to use "usted" with older people to show respect.
We are to use tú with people we are familiar with such as close friends and those our age and also with those that are younger.
(Elaine, I'll answer in Spanish because you're studying it. If you don't understand what I say, just tell me.) Esto que decís acerca de que "usted" se utiliza usa para mostrar respeto es así aquí en Argentina y en varios otros países, diría que en la mayoría de los países hispanoparlantes. No estoy de acuerdo con que el ser o no un profesional determine también el uso de "usted". El uso de este pronombre está determinado por la confianza que uno tiene con la persona con la que está hablando.
Also keep this in mind, if you are my friend and you also know me in a professional setting such as a dental office where I work, you would address me as usted in front of people but use the tú form in a more relaxed atmosphere outside of the office.
Esto no creo que sea así, al menos en Argentina. Los demás nativos seguramente darán su opinión acerca de si es correcto en sus países.
Saludos,
Sextus