1. Maintain a lane. When danced socially, tango is danced in strict circular lanes with couples advancing around the room in a counterclockwise direction (called “line-of-dance”). There may be one or more concentric lanes moving simultaneously. Once in a lane, avoid changing lanes during the dance.
2. Look before backing up. Never step backwards against traffic blindly. Likewise, avoid other movements that cause you or your partner to suddenly occupy space behind you in line-of-dance because the dancer behind you may have already begun advancing into that space.
3. Avoid passing. Tango is not a race. If the dancer in front of you is advancing more slowly than you would like, alter your dance so that it is more circular and less linear. Learn to dance well and happily without much forward advancement.
4. No parking. Standing and chatting with your partner between songs is fine, but keep an awareness of when the couples around you start dancing again and move accordingly. If the other dancers have begun to dance and you wish to continue your conversation, simply step off the floor so you don’t obstruct them.
5. Never zigzag. Cutting in and out of line-of-dance is very poor form and disturbing to the dancers you are cutting in front of. If you choose to dance in the center of the room, remain there throughout the song. If you dance in a given lane, finish the dance in that same lane.
6. Don’t monopolize the space. There are many styles of tango. Some require a relatively large amount of floor space; some require a minimal amount of floor space. All styles are fine under the right conditions. If a floor is crowded, dance small, not taking up any more space than any of your fellow dancers. If the floor is not crowded, and you are so inclined, dance large.
7. Avoid dangerous moves. Certain moves, such as high in-line boleos, can be dangerous on a crowded floor. Save them for less crowded conditions.
8. Don’t talk, dance! Talking while dancing is bad form, reveals the dancer’s lack of presence in the moment, and is distracting to other dancers. Save the conversation for when the music stops. Teaching or correcting your partner is particularly inappropriate at a milonga. Save it for a practica.
9. Dance with the room. Endeavor to dance with an awareness of all of the dancers around you. Do not allow gaps in the line-of-dance in front of you to form as this will cause a pileup of dancers behind you. When the music begins, start dancing when the majority of other dancers do.
10. Ask before merging. Before stepping onto a crowded dance floor, if you are a leader, make eye contact with the leader whom you wish to enter the floor in front of. The leader should understand your request and indicate his assent with a nod or wink, and you may then enter line-of-dance.
From http://www.wasatchtango.org by Daniel Boardman