What form is normally used when talking to people like sales clerks, waiters? (It seems to me that real life Spanish is quite different from what the phrase books say).
I don't know what the phrase books say, but in Latin America, you can't go wrong with
This always comes across as respectful with someone you don't know well.
This next part is some bonus information which is based on cultural norms in Mexico. I can't vouch for other Latin American countries.
It is a good idea to tack on the equivalent of sir or ma'am, as is often done in the South of the United States, to almost anything you are going to say to a restaurant, hotel or store worker:
Señor for a man in general
Señora for a woman in general
Joven for a young man
Señorita for a young woman
Seño for a woman when you're going back and forth between Señora and Señorita
Jovencito, jovencita for a child (for example in a small restaurant or hotel, a child as young as eight might be helping out, e.g. bringing water)
Politeness tip: Be very liberal with your greetings, good-byes, and thanks. The norm for these small polite expressions is much higher than in many parts of the U.S. at least. (Can't say, regarding the UK, etc.)
Observation about the tú/usted dilemma regarding foreigners traveling in Mexico: Many Mexicans automatically make a big shift in their politeness norms with regard to foreigners. They often address foreigners as "tú," in the same way they would address a child as "tú." Thus, just because a stranger in a market is addressing you as "tú" does not necessarily mean that is the cultural norm in that location. It is more reliable to observe how local citizens talk to people they don't know well, to find out what is the norm in a particular location, than to go by the forms of address being used by strangers who are talking to you.