This may sound like a truism, but...
- The most important interview tip: Be yourself.
- Have a positive attitude, don't be nervous, don't be fake.
- Think of the interview as an opportunity for the interviewer and you to know each other.
This is mutually beneficial: the interviewer will learn whether you are fit for the job, and you will get a feel of what life in that company is like.
- Do not try to hide your feelings about the company or the job; they will show anyway. If you feel uncomfortable during the interview, one of two things will happen: 1. You won't get the job, or 2. You will feel just as uncomfortable while doing it. I don't know which one is worse..
- Have fun! The interview should be a fun, interesting experience for both of you. Show them that you'd be proud to work there, and make them feel proud to have you.
- As sockpuppet said below, your inquisitive mind, problem solving abilities and desire to learn new things matter more than your actual knowledge. The interviewer would prefer someone who gives an "I don't know, but it sounds interesting! I want to learn!" answer to someone who tries to cover up his ignorance or lack of understanding.
- Surprise the interviewer. Be it with your knowledge, your wit, your personality, your open mind. This is how you make sure you'll stand out.