I found an old GeoCities website with a list of "correct social behavior" in Spanish culture. It's not incredibly accurate, so I thought I'd reprint it and add my own comments. The first thing I'd like to make clear is that things are really not all that different from America. If you come to Spain and behave politely and in a friendly manner, but in the way that's natural for you, you aren't going to have too many problems. Most Spaniards outside heavily touristed areas are generally pretty tolerant and understanding with foreigners. If you're there on business, understand that your Spanish colleague is going to have learned all kinds of stuff about American business culture and won't be offended at anything you do as long as you're straight with him. Also, your Spanish colleague will most likely know rather better English than you do Spanish, and so you're likely to talk in English.

My comments are within parentheses.

First Name or Title?: Respectfully addressing others in Spain

First names are acceptable for only close friends, children, and teenagers. (First names are now universal in casual conversation.)
When addressing others, follow Spanish business protocol by using the formal usted mode of address unless invited to use the more informal tú. (True for business and older people. Otherwise use tú unless you hear the Spaniard call you usted.)
When addressing each other, men who are university graduates, businessmen or other professionals often use the courtesy title Don to confer respect. (Yeah, if they're eighty-two years old.)
It is important to address individuals by any titles they may have, followed by their surnames. For example, teachers prefer the title Profesor, and engineers are referred to as Ingeniero. (No. That's Latin America. Here everyone is Señor.)
Whenever you can, address people using their professional titles followed by their surnames. (No.) Professional titles are usually not used, however, when addressing Spanish executives. Basic titles of courtesy (followed by a surname) are always appropriate: Mr. = Señor; Mrs. = Señora; Miss = Señorita.

Public Behavior: Acceptable public conduct in Spain

A wide range of gestures regularly accompany conversation. Don't hesitate to ask if you're having difficulty understanding these gestures, especially since the meanings often vary from region to region. (You ought to be able to figure most of them out.)
Spainiards get a sense of identity from their particular region rather than the country as a whole. (NO! This is ONLY true in Catalonia, the Basque Country, and Galicia.)
Be sensitive to regional differences; making misinformed comments about a Spaniard's region of origin is considered a grave insult (i.e, mistaking a Catalan for a Basque). (What would piss a Catalan off is being mistaken for a Madrileño. Seriously, avoid making misinformed comments about anything, of course. Don't worry about this. they'll understand, though you might get a lecture on how Catalans are different or something like that.)
Handshakes are a standard part of Spanish business protocol. (Yep.)
First-time introductions with Spaniards should be made in a formal manner. Extend a brief but firm handshake, while maintaining eye contact during the meeting. A "Buenos dias," "Buenas tardes," or "Buenas noches" should accompany your greeting. (Just like anywhere else.)
Women sometimes lightly embrace, then touch cheeks while lightly kissing the air. You may also observe a professional woman greeting a Spanish man who is a particularly close colleague in this way. (Women ALWAYS do the kiss-kiss thing with one another, and they generally do it with men, too. Don't make the first move but don't be surprised if you are subjected to the kiss-kiss routine.)
In the company of friends, it's common in for men to hug or pat each other on the back, in addition to a handshake. (Again, don't be surprised if this happens, but don't do it yourself.)
Spaniards may not only stand uncomfortably close, but also pat your arm or shoulder when conversing with you. If you encounter these gestures, moving away will only cause offense. (True.)
One common gesture is snapping the hands downward to emphasize a point. (True, though it's one hand, not both.)
The North American "O.K." symbol (i.e. making a circle of the first finger and thumb) is considered vulgar. (No, that's not true here. That's Brazil where it's rude.)
Spain is a highly religious country (not anymore), and many people will be offended if they hear you take the Lord's name in vain. It's also a good policy to refrain from swearing in the presence of others. (Of course you shouldn't swear, but nobody's going to be offended by an "Oh, my God!" or the like. And don't be surprised if your Spanish colleague starts swearing. Spaniards swear like sergeants.)
When summoning a person, turn your palm down, then wave your fingers or entire hand. (You don't have to do that, they understand the equivalent American gesture just fine. If you see some guy flopping his wrist around, though, it doesn't mean that he's exaggeratedly effeminate.)
If you are in a long lineup, don't be surprised if someone tries to cut in front of you. (When you come to the end of a line, say "¿Quién es el último?" Someone will respond. You are now behind that person and have rights to your place in line.)
Be aware that while Spanish men can be very charming around women, their approaches may be too forward for some people's tastes. For example, when they see an attractive woman walking down the street, they may whistle at her to signal their approval. (If they're construction workers. Respectable people don't do that.)
Before getting into a taxi, be sure to negotiate the fare. (Wrong. The great majority of cabbies are legit, and fares are metered.)
When a public restroom is needed, men should look for a door marked Caballeros, while women should look for a door marked Señoras. In smaller towns, rooms marked "W.C." (for "water closet") are bathrooms used by both men and women. (They often have symbols.)

Business Dress: Guidelines for business dress in Spain

Spaniards are extremely conscious of dress and will perceive your appearance as an indication of your social standing and relative success. (True, in business, and surprisingly true in everyday life.)
Keep in mind that Spaniards typically dress more conservatively than Americans and frequently wear designer clothes. (True.)
Stick with quality, conservative clothing in subdued colours. Name brands will be noticed. (True.)
Dressing con elegancia means that men should wear dark suits and ties, preferably with starched white shirts. Suit jackets should be kept on at all times, unless your Spanish counterpart invites you to do otherwise. (The shirt doesn't have to be starched white. It should be appropriate for business wear in the US.)
Women should dress with elegance and style. The best clothing options for female business travelers include designer suits or business dresses made of high-quality fabrics. (True. Don't be surprised if you see Spanish women wearing sexy clothes that would be inappropriate for business in the US, though. I wouldn't do that myself if I were you.)
Pantsuits for women haven't gained a lot of acceptance here, but wearing "dressy" pants in the evening is fine. (True.)
Shorts are unacceptable in public. (In business, of course; no longer in everyday life, at least if they're conservative khaki or navy Bermudas)