Before you can possibly convince the interviewer that you are right for the job, you have to believe it yourself. It's amazing how many candidates seem tentative and reluctant to express personal confidence in their own abilities. Remember, you are all alone once the interview starts. No one will sell you if you don't sell yourself. How can the employer believe in you if you don't believe in you? The interviewer is not there to sell you on their company until after you sell them. Once you have sold them on you, they will sell you on the position and the company, but not until then. So don't expect the interviewer to tell you why you are right for the job. That is your responsibility.
The night before the interview, spend some time with a friend or family member, telling them why you would be the best for the position. Use superlatives galore! The purpose is to put you in the right mental state for the interview, so that you truly believe you are the best possible candidate for the job. You need to prep yourself mentally for the task ahead. Interviewing is pretty much the only human activity (OK, maybe dating is the other) where you spend all of your time trying to convince someone else you are awesome. So you need to get your head into the game for the challenge that is ahead. Don't go into the interview cold, expecting the words to flow. You need to practice in advance.
So maybe you are the shy type who is uncomfortable talking about yourself in a positive way. There is still a way for you to prepare yourself mentally for the interview. Remember Pygmalion? In Greek mythology, Pygmalion sculpted a beautiful ivory statue of a woman that was given to the king of Cyprus. Pygmalion believed so strongly that the statue was real that it was eventually given life by the goddess Aphrodite. If others tell you that you can do something, and tell you this long enough, you will eventually come to believe it yourself and live it in your life.
To see a simple example of the power of this technique in action, notice what happens to you when you smile for an extended period of time. Think of something (or someone) pleasant or amusing that makes you want to smile. Right now, as you are reading these words, just do it. And hold that smile until you finish reading this technique. The end result will be that your body will react to the smile in a very positive way. You will eventually feel like smiling naturally without having to consciously think about it. And, as a secondary effect, if others walk by while you have that grin on your face, they will probably begin smiling as well.
We create images in our mind of how things should be. If these images are believed, they can eventually become self-fulfilling prophecies. If we change the image, we change the result. So if others tell you that you are the very best person for the job long enough and sincerely enough, you will eventually come to believe this and act upon it in a positive way.
No, this is not some useless psychobabble—it really works. The key is to pick someone as your supporter who is very sensitive and willing to back you in your efforts. Significant others work great, assuming the relationship is supportive. Moms and Dads are also great for this role. Let your supporter in on the fact that you have an interview coming up, and tell them you need their help in pumping you up. Ask them to please lay it on thick, with the best praise they can muster for the occasion. This should be the last person you speak with the night before or even the day of the interview, if possible.
One final note. This is also a simple, yet very effective child-rearing technique for later in life. Tell your kids they are loved and wanted and they will believe you. Tell them they are hated and worthless and they will also believe you. Make sure you do the former.
The use of mental visualization can be extremely helpful in preparing for your interview. You can, by visualization, experience your coming interview, including a rehearsal of how you would react in specific situations.
Many great athletes prepare for competition through visualization and imagery. And many of the great feats of history have been accomplished first through visualization. If you watch closely in the starting blocks of any competitive sport, you will see the athletes using visualization to take them through the entire race in advance. Many countries, understanding the benefits of visualization and imagery, employ sports psychologists to support their teams at the Olympics.
In preparing for the interview, go through the motions in advance in your mind. Anticipate the questions that may be asked. Visualize yourself as confident and self-assured. Not cocky, just confident in your background and the benefits you can provide the employer. Play the part over and over again until you feel you have truly lived it. Visualize your success until it becomes reality.