Times are tough. Jobs are scarce and there are more people vying for each position than there have been in a long time. If you're employed, it never hurts to be prepared for the worst, and if you're looking, hopefully this will give you an extra edge. Here's my advice, from a lady who hires freelancers and contractual artists on a routine basis.
First off, congratulations! You got the interview! Which means you must have done things right, had an excellent resume, well put and memorable cover letter and now you potential boss wants to meet you in person. Feel really good about this, allow it to soak up all your insecurities about yourself and your job prospects. Ah, feels good yeah? Okay, now back to reality. If you're going to get this job you're going to have to do some more work, and dazzle them in person the same way you did on paper.
This one should be easy since your dazzling cover letter reflected their company's goals and interests right? If you did, review your cover letter and your sources to refresh your memory. If you didn't, count your lucky stars and start now. Find out what the position clearly does so you can sell yourself completely to that niche. If you know who your interviewer is, find out what their job entails and familiarize yourself with the department in general. Make sure you know your resume and your cover letter intimately, as they'll probably have it in front of them and you want to know what they're talking about if they bring up something mentioned in either. This is the easiest thing you can do to help your confidence and make you just a little more shiny than the competition.
You want to be professional, but not boring. If you're male, wear a tie, even if you think it will be too formal. Always overdress rather than underdress, you can tone down your wardrobe once you get the job. Ladies, if you're wearing a skirt, make sure it's around knee length, too short and you'll be seen as slutty, too long and you'll be dowdy. Wear tights or stockings with your skirt, and always bring a spare. Murphy's Law says they'll ladder right before your interview. Make sure your hair is clean and as frizz-less as possible. Don't wear ostentatious accessories, but do add a bright blue brooch to that gray suit or a striking maroon tie with your white shirt. Put a pop of color in your outfit and you'll be more memorable without having to do much work.
As a subset, continue to carry yourself well while in the interview. Don't slouch, work those pearly whites and keep your body language warm and open (ie, no crossed arms).
This is one time where 15 minutes early is on time. Be there half an hour early at the earliest though, otherwise you may look desperate and interrupt your interviewer's schedule. If, like me, you tend to get lost the first time you go anywhere, leave an hour, two hours, whatever it takes, early. Hang out at a coffee shop before your interview if you're super early. Just don't be late. If by some chance a semi lights on fire and you're stuck in the aftermath of the fire department for half an hour, make sure you have the phone number to call. Explain what's happening and tell them an anticipated arrival time and ask if it would be better to reschedule. Don't lie either; it's easy enough for them to look up a traffic or bad weather report where you are.
You can over-prepare, so make sure you have a general overview, but not memorized answers. You want to sound well-informed, not like a robot. An interview is not a test, it's an assessment of you, your knowledge, personality, and poise all in a very short, pressure-filled atmosphere. Be ready to answer questions you might not be prepared for, and don't worry so much about having the "right" answer as you do having the right attitude. If you get stuck or need a minute to think, say so with a "That's a good question . . ." or "I need just a second on that one . . ." They'll appreciate a well-thought-out answer more than a rushed one.
Tell them a story
This is the movie of your life, starring you. Make it interesting and engaging. Set a scene and play it out, instead of listing all your job duties, talk about how the one time you were given a last minute assignment and pulled it off with ease, turning it in half an hour early. Engage your listener, the last thing they want is a boring interview, which is likely how most of their other interviewees are. Be different! Do not ramble though, make sure your stories connect with the interviewer's question and illustrate what a good worker you are. Brevity and clarity are your friends.
Turn on the Charm
Smile when you introduce yourself, look your interviewer in the eyes, and give a good, firm handshake. Think about your interview as a first date. You are flirting with your interviewer. Don't be too forward or too coy. Make yourself approachable, but not desperate. Read your interviewer to see how they want you to react. Take note when they smile, and what makes them sit up and take a closer interest in you. Take all that you can from those clues. If you're in a situation where your interviewer is stone-faced, don't freak out. They're trying to see how you'll do in a tough situation; show them it'll be a breeze for your shining self. Be enthusiastic about learning how to do the position if you're a little underqualified, or in a new field. Remember to keep your calm even if it feels like the whole room can hear your heart pounding.
Be ready to ask questions
While not always applicable, most job interviews I've had have ended with "Do you have any questions for me?" Don't just mumble a "well I think you covered most of it." Be ready to have a question that makes them think you know what you're doing. If you've done your research you should figure out a question that will show both that you know how the system works, and that you're interested in their take on it. Questions about the interviewer's role and position will are also generally appreciated.
This is I think one of the most overlooked parts of the interview process, and one that can give you a big leg-up on the competition. The day of, or day after, the interview, send a short email thanking the interviewer for their time, the opportunity to present yourself, and the conversation. Don't repeat what you said in the interview, but if there was a short point you forgot to make, do it discreetly now. Don't be pushy, but showing that you care and keeping yourself fresh in the interviewers mind might make you that new person over someone who did just as well but didn't bother to check in.
How do you prepare for an interview?