Before the interview
Prepare for your interview: Learn everything you can about the company. Check out its website. Explore it like you would look for clues and find out what its ethos is. Then find a way to work this into your speech. Pick three or four recurrent words and try to use them during the interview.
Prepare questions: The interviewer will most likely ask if you have any. If you don't, it makes you look naive. Google your name: Companies do a Web check before inviting you to an interview. You don't want embarrassing photographs or text out on the Net.
Prepare any necessary papers: resume (e-mail it before the interview and bring a printed version with you), reference letters, etc.
On the day: Confirm your appointment if it was made a few weeks before and make sure you know the right address and time for the meeting. Also, have the full name and contact of the person you're meeting. Bring a notepad and pen in case you need to take notes.
Dress appropriately: If you can, check out how the people who already work at the company dress. It's always better to be overdressed but if you're applying for a job at a casual restaurant, you might want to forget about your Armani suit.
Tidy up: Wearing a suit doesn't exempt you from shaving and brushing your hair. Smell of nothing but perfume (do not smell of cigarette smoke when you go for an interview). Make sure your breath is minty fresh, but do not chew gum while waiting for or during your interview.
Plan your trip: Arrive 10 to 15 minutes early. If you don't know the area, print a map before you go.
Turn off: your mobile phone.
During the interview
If you've followed the steps, there is no reason why you should be stressed. Try to be natural: it's an interview, not an execution.
Give your interviewer a firm hand shake.
Be careful of talking too much. Get directly to the point.
Stay professional even if the atmosphere is relaxed. Watch your language, manners, body language (sit straight and still, maintain eye contact).
Stay calm, even if you think you're not doing well.
If you're surprised by a question, don't rush to answer. Take the time to pause before answering.
If you don't know what the interviewer is talking about, ask. It is quite alright not to know the answer to something.
Let the interviewer speak. Listen to what he has to say.
Be positive and energetic.
Whatever you do, don't lie. You will be caught out and it can be very embarrassing later on.
Ask questions that show your interest and your understanding of the position and capacity for initiatives.
Don't try to portray yourself as a person that never makes mistakes. Give an example of how you learned from your mistake and how you have benefited from that experience.
At the end of the interview, make sure you understand what the next step is.
After the interview
Follow up the interview with an e-mail on the same or next day.
Don't be too pushy to know if you're hired or not. If they said they would call in a week, wait.
If you're not successful, say you were happy to meet them and would be happy to be interviewed again if there is another opening. And don't be depressed. Interviewing is about finding the right person for a job, and that's not always the candidate with the most experience, education and qualities.