Always greet Central Americans with a cheerful buenos días in the morning and buenos tardes in the afternoon. Excuse yourself from company by saying "permiso." Don't get too hung up on whether you address people formally (usted) or informally (tu or vos). Most locals make allowances for the fact that you are a foreigner, speaking a strange tongue, and won't get offended by such subtleties. Medical professionals like to be called "Doctora" and it is always wise to address a policeman as "Señor Policia."
Also be careful with your hand gestures. Central Americans use gestures that are often the opposite of what you may be used to. For example, a beckoning index finger is regarded as vulgar. A downward shooing gesture actually means "come here!" The universal finger wag is however the same everywhere and can be used in all sorts of situations from haggling to arguments. In addition, Central Americans are not as outwardly affectionate as their South American cousins. Females are sometimes kissed on the cheek, but if in doubt, a handshake will suffice.
Most Central Americans dress in a conservative manner; this is less true for the younger generation who are more casually fashionable. The torpid weather will compel you to wear light clothes and shorts (and that's perfectly fine in most restaurants and attractions), but be aware that this mode of dress is not acceptable in churches. Also, while most Maya craftspeople are more than happy to see foreigners purchase their goods, for some indigenous people, seeing tourists walking the streets in native garb can be insulting -- especially when women unknowingly wear traditional men's clothing, or vice versa. Use caution, and when in doubt, don't model your purchases in any but the most touristy towns or settings until you get home.
Note: This information was accurate when it was published, but can change without notice. Please be sure to confirm all rates and details directly with the companies in question before planning your trip.