Lonely Planet seems to have the best laid out website and information, and I thought it would be fun to share some info I came across that gives fascinating insight into their culture:
shake hands and say mucho gusto (nice to meet you) the first time you meet someone
preface your conversation - even simple requests - with buenos días (good morning) or buenas tardes (good afternoon/evening). It is also courteous to say hello to the person sitting next to you on the bus and to make a general greeting when entering a public place like a restaurant.
use the formal usted (you) to address locals until they go to the informal address first. In El Salvador, that’s the vos form - the same tense used in Argentina instead of tú. Take the time to learn it before you go. And needless to say, have at least some basic Spanish under your belt.
bring mementos and souvenirs from home to give as gifts to special folks who have helped you or invited you to their home
pay attention to your appearance. Salvadorans are very conscious of appearance, grooming and dress. Looking scruffy and unwashed is considered an affront.
go into shops shirtless or in a bikini. Though the beach may be nearby, wandering around in such skimpy attire is inappropriate. In general, steer toward conservative attire, especially when visiting churches, where you should avoid wearing tank tops, shorts, or hats. No Salvadoran wears shorts outside of the beach and coastal towns.
refer to indigenous people as indios, which is considered an offensive term. The word indígena for indigenous men and women is widely used.
expect everything to rush at New York City pace
take photographs of religious ceremonies or people without asking
be insulted if people comment frankly on your physical appearance or give you a related nickname (e.g., El Gordito/Fatty). This is done affectionately and isn’t meant to be hurtful.