Wear comfortable shoes that will stay on your feet when you spin. Women, do not wear strapless sandals. They will fall off. Men, don't wear boots, you won't be able to turn quickly.
Ponytails are vicious on the dance floor. If your hair is long, let it down or put it up.
Say yes when asked to dance. This goes for both men and women! Relax, you don't have to marry this person. If you really don't want to dance right now, say you are sitting the song out— and then sit the song out. After dancing once with a particular person that night, it's absolutely OK to say no.
Take dance classes, and dance socially with a variety of partners. Both are very important. Especially for women, because men will lead you in a variety of moves and lead you in a variety of ways.
Private lessons can improve your game quite a bit very quickly.
Support the venue— buy a drink! Buy soft drinks or water if you don't drink alcohol. I do not recommend having more than one drink before a lesson.
Women: relax, don't grip his hands and let him lead you. It's good to provide tension but this is not a wrestling match— a little feedback goes a long way. Men: lead! Don't wait for her to direct you. If you want to alter the ground rules, ask first.
Casual means no T-shirts, shorts, sneakers or boots. You might get away with a good-looking T-shirt, but nothing with words on it please.
Date means all of the above, plus no jeans. You can't go wrong by dressing better.
Calle pero elegante ("street but elegant") is classy casual— jeans OK but make an effort!
Dancing is an event. You've been looking forward to this. Dress accordingly.