A lot of otherwise competent people get scared when having to face a job interview. That makes sense: It's an uncommon situation in which people have to answer questions about themselves, with high stakes attached to the answers. And even people who are otherwise very good at talking can get tight-lipped when it comes to interviews; there's a lot riding on what they say, and many interviewers ask questions designed to trip people up.
But it's possible to put your nerves mostly aside. A job interview does not have to be scary if you do your work well beforehand.
Take every interview
Back in my days of looking for jobs, I went on every interview offered, even when the jobs seemed unappealing. Sometimes this led to pleasant surprises, when I actually decided I would at least consider the position. In other cases, low-stakes interviews for jobs I didn't want gave me added experience in interviewing.
Before going to a job interview, prepare a dossier about the company, the job, the person interviewing you, and anything else that might come up. Have some notes on how you would approach the job (being careful to not sound arrogant), and be ready to talk about situations that might arise.
Approach the interview the way an assistant coach goes after a head coaching job. In those cases, the coach is ready to lay out his or her philosophy, name some assistants he or she might hire, and discuss every tiny detail that might come up.
You may never be asked for any of these things, but having them ready will prepare you for any question. It's better to go in overprepared than to be surprised.
Do your research
For bigger companies, there is often information online about the interview process. Sometimes this just lets you know what to expect; in other cases, actual questions will be shared.
Do the research, but don't put all your faith in having the answers before the test. Remember that things can change.
Grow a modest ego
A job interview is an opportunity to sell yourself. That's a challenge for people who are better at deflecting or directing praise to others.
Be prepared to both tout and explain your accomplishments. Saying that you raised sales by 10% is one thing, but telling the story of how you did it is more valuable. Don't brag; be analytical, while also taking pride in your accomplishments.
Relax and breathe
When going to a job interview, be confident that you are the right person. And remember that the company has to sell you on the job as well.
That may not be true -- you may desperately need or want the job -- but don't let it show. Confidence and preparation can cover shyness or other faults. If you've put the work in, that will be remembered more than the fact that you paused a little too long when asked a question, or that you spoke a little fast in response to another.
Remember that the person or people on the other side of the table have most likely sat where you sat. Show them who you are, and that you take the process seriously. If you do that, then everything else should fall into place.