This sounds really silly that smiling makes a huge difference, but I'm convinced it does. It just shows the interviewer that you're a relaxed, happy person who realistically is going to benefit them by hiring you. Plus it makes you more approachable and if you do get remembered for smiling loads that can't be such a bad thing....
Do not be a field mouse. Stand up, be bold and walk in with your head held high as if you can take the world. Chances are, you can.
Have an neat, easy to follow copy of your CV. There is nothing worse than a CV that you can't follow - infact most managers just won't bother. Turning up on time - if not early - gives the potential employer the idea that you are prompt and totally worth their time. Be prepared for the questions that they will ask you, about yourself and how you can benefit the company. Plan ahead.. which leads on to....
Remember that you are trying to sell yourself in the best way that you can. Pick clothes that are smart and show you off to the best of your ability. Don't wear scruffy clothes and make an effort to not look unneat. It has also been suggested that you don't wear just black - a bright colour will stand out.
Perhaps the most important of all. Being yourself. There is no point pretending to be something that you are not just to get a job, else you will struggle if you manage to secure a job based on the fake you. You will get the job if you honestly are a fit for the job itself.