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 on this page. 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, interestingly enough, if others walk by while you have that silly 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 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 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.