Generally speaking wear something smart. If you don't you may give an impression of someone who couldn't be bothered.
Prepare to the interview, have a good idea of what the firm does and what the job you are applying for entails, and what you can offer the company that makes you the one they should choose.
Take a deep breath when you need to. A little hesitation to think about an answer is ok, folk are aware interviews are nerve wracking, but naturally too long a pause doesn't give the best impression.
Pretty much be yourself. The individual interviewing you is only human, they do the same things you do, and may even be nervous of holding an interview, so no need to concentrate on your self all the time, try to relax and so appear a good colleague to work with.
Get a decent night's sleep beforehand, so you are fresh and raring to go.