Dress for the job you want to be hired for. If you are applying at an office or corporate building, dress smart. Blazer, blouse, clean and tidy. The works. Wear a confident smile and a know-how-can-do attitude. My How to may help you with dressing: http://www.funadvice.com/howto/dress_professionally_job_interview
Make sure you bring your CV, even if you have already sent it to them, just for reference. (http://www.funadvice.com/howto/write)
Speak professionally, and don't say anything that doesn't need to be said. Eg: 'OmGgG we had these coffee machines at my old job! Me and Mark always used to sneak coffee when the boss wasn't looking! LAWL WE WERE LOLAGE'. Be polite, but not a suck up. Don't go in yelling all of your qualifications in the interviewer's ear. Be humble, but make sure the interviewer knows what you can do (another reason to bring a hard copy, or two, of your CV). Don't be in a rush or have to leave before the interview is over. That's an instant NOT HIRED.
Just remember to be confident in your work at all times.
Best of luck.
One thing nobody mentioned (in the answers, at least)...wearing glasses. Even if you don't actually *need* them and have great eyesight, studies have shown that people with glasses are perceived as more intelligent ...so if the job requires intelligence, it's one strategy to consider. Post hire - just tell them you got contacts.
Also, according to the wall street journal...more than half the people who get a job lied on their resume...so, stretch the truth :)
Of course...these are ideas bordering on the morally questionable so, well, don't do them if that's not your thing.
Good personal hygiene, professional appearance, strong handshake, knowledge of the company, a friendly smile, being prepared, making eye contact while talking, experience, willingness and eagerness to learn, being polite and respectful, and having confidence in yourself.
Making a good first impression.
You should look neat and well put together, smell nice, speak well, have confidence, be honest, etc...
Check out my How To, it may help - http://www.funadvice.com/howto/introduce_job_interview_good_impression
i hate to say it :\ wearing a short skirt :( actualy i think it depends on teh job try being really polite as well