Before you enter the interview room, you are just another name on the list. Make sure that by the time you come out, yours is the only name on it. Remember, it's always better to get the job offer, then decide if it's right for you. You can't have that option if you aren't offered the job. Stay calm, think clearly and don't let tricky questions and being under pressure put you off your stride. Convince the interviewer that you are the best possible person for the job.
Preparation
Make sure you know exactly where the interview will take place and the name and position of the person you are due to meet. Work out your route in advance and allow a generous margin of error in case of unforeseen circumstances. Have the telephone number of the company handy so that you can let them know if you are running late - this is generally forgiven where as unannounced lateness generally isn't. Find out roughly how long the interview will take and arrange other appointments accordingly.
Research
Where we can, we include company information to help you prepare for your interview and better understand the company and the work it produces. However, you may find that digging a little deeper also boosts your confidence at the interview. A Company website is a potentially excellent source of suitable information. Being even a little knowledgeable demonstrates to the interviewer that you have gone to the trouble to see whether you'd fit into the team and make a successful 'career' out of a 'job'. Make sure you ask intelligent questions, but not of the 'how much holiday' variety!
Quiz Yourself
Time and again interviewers ask the same general questions in addition to those of a more technical nature. Learn them, prepare your answers, and practice them on friends. Common questions are "Why are you interested in this position?" "Tell me about your current boss". "What are the most satisfying/frustrating things about your current employment?" "What are your strengths/where do you think your weaknesses lie?" "Why do you think we should give you this job? "Where do you see yourself in 5/10 years time?"
Dress to Impress
People shouldn't judge a book by its cover, but they do, all the time. If you look untidy that's how you'll be remembered and first impressions really do count. Always wear a suit and tie or neat skirt/dress and polish your shoes. Even if you are attending a second interview and you know the organisation allows less formal work dress, maintain a professional/formal approach, as you don't know who you may happen to meet! Shake hands firmly with people you are introduced to and when you leave.
Be Unaggressively Assertive
Find the right tone in which to present your positive aspects. You have to talk about your achievements to show you are the right person but do it without being boastful. Under no circumstances should you spend a significant portion of the interview running down your current employer - this is viewed as negative.
Turn Weaknesses into Strengths
Don't pretend you have no weaknesses because everyone has some. On the other hand don't put your self down. Discuss your weaknesses as though you have recognised them and strive to overcome them. "I used to have bad time management, now I prioritise my workload first thing every morning". "I like to take control and be involved in everything. I sometimes find it hard to delegate but when I have the results have been positive."
Positive Mental Attitude!
Always go in with the intention of getting an offer, only then do you really have the chance to weigh up how this opportunity compares with others. Many people, with the benefit of hindsight, have regretted they didn't take a particular interview sufficiently seriously.