When you go to an interview, think about the guys on the other side, the interviewers. Imagine they are also under stress, that they are having a hard time finding a decent candidate, they are scared they won't be able to do their job properly, or that their lack of knowledge on the subject will show through, even though they're the ones asking the questions. Try to see the interviewers as human beings, find some natural point of empathy with them (children? a bad cough? traffic difficulties ?) unrelated to the interview. If there is more than one, concentrate on whoever you feel most affinity with.
Even if your technical knowledge is limited, concentrate on your strengths: practical experience, a positive attitude, fluent communication, a good fit with the company ethos, a positive contribution to the working environment, etc. That's two thirds of it right there; it's not all technical. Investigate the company and make sure you understand what distinguishes it from the competition in terms of company culture. Try to understand the company's vision statement and what it entails.
Also think that you have a broad choice of jobs on the market, that these guys have to convince you to work for them: they have to show they are offering a good working environment, decent career prospects, a competitive salary package. Be the opposite of desperate: you're in a strong position, you have all the choices, they have to entice you.
Even if I can't answer all their questions. I generally behave as if I were a very valuable potential asset for the company, which they will have to work hard to acquire. I certainly don't think they are doing me any kind of a favour by offering a job: how would their company function without decent workers?? If I have any doubts about the company, its training policy, its promotions, its financial viability, I ask them, I put the interviewers on the defensive, so they try to convince me that it is worth my while to work for them.
When you get to the financial part, don't sell yourself short, and don't look greedy either. I find that a 10% raise on my previous package usually strikes the right balance. Sometimes you can ask for 15%, and then let the guys negotiate it down to 10%, so they have the impression they got a bargain.
Usually this approach catches interviewers on the wrong foot and reverses the roles. I get a lot of offers, even though I don't have a degree in IT, and I'm an average programmer (I studied philosophy).
To sum up:
- be empathic with the interviewer
- be positive about your market prospects
- have a deep understanding of this particular company's vision, why you want to work for them, what positive contribution you can make
- be upfront if you have any qualms about the company or the role
- be clear about your strengths and know that you are not being judged exclusively on your technical abilities
- make the interviewers work to convince you to sign a contract