1. Remember that the employers thinks you're qualified. The fact that the employer invited you to interview means that they have already determined that there's a very good chance that you might be the best person for the job. If they didn't think you had the basic qualifications, they would have called other candidates instead.
2. Remember that they've never seen a "perfect" candidate. When you're nervous about an interview, it's easy to imagine that the other candidates for the job – your competition – are perfectly qualified and giving flawless interviews. But no candidate is perfect and most people don't give perfect interviews. In fact, perfection isn't the standard you need to strive for. Just aim to give a good interview that conveys a sense of what you'd be like to work with day to day.
3. Know that you have some control here too. Job seekers often feel as if an interview is a one-way transaction, where they just wait for a company to pass judgment. It's important to remember that you have power in this situation too; part of the point of the interview is for you to do your own due diligence and decide if you even want this job (or this manager or this employer).
As a side benefit, approaching an interview this way will make you a more attractive candidate. When interviewers can tell that a candidate is interviewing them right back – not just hoping for an offer without truly considering whether or not this job is right for them – it makes the candidate seem like someone with options, which makes them more desirable.
4. Remember that this is a business transaction. When you go to an interview, think of yourself as a consultant with a service for sale (your work) and the employer as a potential business partner who might be interested in purchasing that service. Approach the meeting just like a consultant would – as a collaboration where you're trying to figure out if working together makes sense, not an interrogation by someone who holds all the cards.
5. Pretend that you already know that you're not going to get the job. How often have you heard people say that their best interviews were the ones they weren't that invested in? Use this to pull a mind trick on yourself: Pretend that someone else has already been selected to fill the job (the boss's kid, an internal candidate, etc.) and they're interviewing you because they have to talk to their top three candidates anyway. This works because it means that nothing is on the line, and their decision won't be a reflection on you. And as a result, you might perform a lot better.
6. And if all else fails and you're feeling like a complete mess inside, remember that it's probably not obvious to your interviewer. First, lots of people in interviews seem nervous to some extent, and interviewers are used to it. So for nerves to stand out, they have to be particularly unusual – such as uncontrollable shaking, or being so paralyzed by anxiety that you can't answer questions. This is extremely rare, so it's probably not the case with you. (And if it is, any normal person would feel understanding and compassion toward you anyway, rather than judgmental or snarky.) What's more likely is that you seem pretty normal on the surface and you're the only one who knows that your stomach is a churning pit of despair.
Alison Green writes the popular Ask a Manager blog, where she dispenses advice on career, job search, and management issues. She's also the co-author of Managing to Change the World: The Nonprofit Manager's Guide to Getting Results, and former chief of staff of a successful nonprofit organization, where she oversaw day-to-day staff management, hiring, firing, and employee development.