As a consultant, I constantly have to update my resume and I go through an interview process for each client. I've also done the unemployment thing, so I've got far more experience with this than I'd like to.
Some of these are my own observations, and others are advice people gave me that actually worked.
Without further ado...
Don't stress over technical tests. It says a lot about the culture of the company and they are a total crap shoot. For what it's worth, I have 7 years in the industry and have won accolades, been given pretty large raises, promotions, and have many positive performance and project reviews. I have never passed a single written exam at an interview. I am still a top producer on every team I've ever joined. Those tests say more about rote memorization than it does ability. Failing one of those tests is not an indictment on your ability. It is not worth getting discouraged over.
When someone asks a technical questions, answer confidentally, but honestly. Saying "I don't know" or "I'm not familiar with that" is better than trying to bluff your way through a question. Even if you get a question wrong, if you seem positive about your answer the interviewer might follow up to determine what your thinking was behind that answer.
Smile, dammit. Look like you are glad to be there.
DO...NOT...WHINE! No one wants to hear your sob story, do NOT talk about how bad your last boss was. Most hiring managers want people with a positive outlook. It makes it more fun to break your spirit later.
Do not be afraid to talk, but do not dominate the conversation. My rule is 40/60. In other words, talk 40% of the time, let them do the other 60%. If things start to lull, ask questions and direct the conversation into areas you are comfortable with. This is very important though, the more talking you do, the more likely you will say something that is going to work against you. Some people, once they get going, can't stop themselves. At the same time, an interview is basically a chance for the prospective employer to get a feel for you. If they end up doing all the talking, they'll come away with nothing. Also, the 40/60 rule should be spread out across the interview. Do not talk for the first 40% of the interview or the last 40%. It should be give-and-take.
Tell stories. Everyone says they "learn fast" or "work hard". Illustrate your skills by giving examples or past accomplishments or challenges that you successfully overcame.
BEFORE you go into an interview think about this question - "What areas do you need to improve?" or similar. This is the absolutely worst question to go into cold. Most interviewers are going to ask this question. If you think about it ahead of time you are more likely to word it in a manner that won't scare them off.
Be flexible. I went into a technical interview ready to talk about my last project and the interviewer had an old resume with my law enforcement experience still on it and wanted to talk about my time working in the jail. Believe it or not, I managed to tie that into my experience as a programmer and used it to explain my career transition. That sort of surprise can be a disaster for some people. Don't assume the interview is going to go in any particular direction. Again, don't be afraid to ask questions or make observations that direct the interview into a comfortable area.
Don't fidget, try to avoid looking nervous. Do away with anything that might be a distraction. Do you have a bulky set of keys? A cell-phone? A PDA? Leave everything in the glovebox of your car except for the key that will let you unlock it. Basically get rid of anything that potentially takes the attention off of you. Even if a cell-phone doesn't ring, it can be distracting. The interviewer will be noticing things about you, so unnecessary bulges, items on your belt, etc. are unwanted.
This should go without saying, but clean-up beforehand. Even if you have a beard, trim it before the interview. Wash and comb your hair. Make sure your hands are clean, including under your nails. Check your appearance before going into the interview and make adjustments as necessary.
Read up on the company beforehand. Memorize some facts, get a feel for their culture. It gives them the impression that you are genuinely interested in them and not just a job. Obviously you should focus on positive aspects of the company.
Have a firm handshake. Not necessarily a crushing one. Give the impression that you are a strong and confident individual. Ladies, no excuses, this applies to you to. You don't have to break any fingers, but don't make it seem like they're shaking a dead fish.
Follow-up, especially if you thought the interview was positive. Something might open up and they'll keep you in mind if they know you are interested. Even if you end up getting rejected, occasionally they might provide some feedback.
Be honest with yourself. Give yourself an honest assessment after the interview and ask yourself what you thought worked and what didn't. Don't oversell yourself in your own mind, but beating yourself up is counter-productive to.
I walk into interviews after going through the company's business and background. That will make a big difference in most cases. I know several candidates that were rejected because they remembered what salary was offered by the company but didnt know what the company did, what domain it was under and who its clients were.
One thing I will add: rehearse. You will get questions like "what did you do at your last job?" "why did you leave your last job" "what are your weaknesses?" etc. Jason correctly points out that you should think about answers ahead of time. You should take it a step further and rehearse those answers.
Think about giving a speech. Obviously you think of what you're going to say before you go on stage. But you probably spent some tie rehearsing what you're going to say and practiced it over and over. I often practice when I'm in the shower (you sometimes need a break from designing solutions there all the time), waiting for the subway/bus, making dinner, etc.
Need better: you're not giving a speech, and a good speech doesn't sound like a speech, it sounds like conversation. Don't rehearse something word for word over and over again. It sounds wooden and rehearsed, and if the interviewer jumps in with a question, it will throw you off. Just as a good speaker doesn't have everything rehearsed word for word, but rather has the message and key points and can deliver them six different ways, you should know what points you want to convey in your story and how to say it many different ways.
Mark Herschberg, author of The Career Toolkit