First, you need to understand the purpose of a technical interview: to gauge and understand a candidates level of technical expertise. Most technical interviewers don't expect you to know everything, but will ask you questions of varying difficulty to measure your technical knowledge and experience. There are some awful technical interviewers out there, so you have to watch out for them, but for the most part, technical interviewers just want to help the company find the best qualified candidate for the position.
1) If possible, make sure you ask about the interviewer. Ask for what their current role is and for a little of their background. This will calm your nerves.
2) If you don't know the answer, say you don't know it! One of the biggest mistakes people make during a technical interview is trying to make up an answer for every question. If you don't know the answer, don't be afraid to admit it.
3) Always give a path to answer the question. One of the things the interviewers are looking for is how the candidate does trouble shooting and what process they use to find answers. If you don't know the answer to a question, then tell the interviewer how you would find the answer.
For exampe, if you didn't know a certain development process, you would tell the interviewer that you would speak with a senior engineer or search for the answer on Google.
Saying "I don't know" and not providing a method of finding the answer is never a good answer!
4) Be careful of questions with no answer. If you encounter a question which seemingly has no end, take the interviewer as far down the troubleshooting path as you have knowledge of and then tell them how you would find the answer.
5) Be willing to ask questions. When you're troubleshooting a problem with the interviewer, don't be afraid to ask them questions to narrow down the issue. Often times, the interviewer has a real situation in mind and is willing to provide tremendous detail to help you troubleshoot.