hey mistyarthur
I work for an airline that is based on easyjets model, and I wore a suit to my interview. Just wear something that you feel comfortable in, but dont be too casual. There were a few people at my interview in jeans, and they did not get through.
Im assuming you are male, (if not sorry!) but for girls, for a low cost airline, You could wear your hair in a (neat) ponytail if you wanted to, and you could wear trousers. For a more formal airline (eg: Gulf Air, Qantas) I would definately wear full uniform shebang.
Try to relax and enjoy the interview, I know its hard, but if you are nervous, it makes it hard to be natural. Also, Im sure you know this, but if you get through to the next rounds, (I sure hope you do!!) keep up your standard of dress the whole way through. At my interview (i found out this later after I got the job) there was a current cabin crew member blending in with candidates in the lobby making mental notes on peoples grooming, and his recommendations were used as part of the recruitement process.
Remember, everthing you do throughout the interview process is being watched!!