One of the things that announces to everyone that you are a "newbie" is to wear a strapless dress. Invariably, the woman spends her time tugging it up, and she typically limits her movement in order to keep her dress up.
A lot of people discount the following, but my suggestion is to wear something that has the right kind of "weighted movement" - a leotard and a character skirt would work. And I DON'T mean some tired, worn-out set of practice wear. You can get some fresh and interesting selections at ballet shops or at online dancewear sites. There are some sites that specialize in ballroom/latin practice/performance wear, if you have a slightly larger budget.