Assumptions
You submitted your resume (have a great resume, I cannot emphasize this enough) and a cover letter to a job, you have followed-up with a call a week later, and landed an interview. Now you need to perform at the interview itself. But what do you wear? How do you act? What about that mole on the tip of your nose?
Prepare
You will be asked questions about how you think you would fit the position, so read about the job in detail. Asking questions about the position is a great way to take control of the interview and let them know you are interested. I am not talking about the ordinary questions. Ask them what a typical workday is like from the time you arrive to when you might leave. Ask them to show you where you would be working. After all, if you are going to be sitting there for eight or more hours every day, it might be nice to know if you have any windows nearby or not and who else is around you. Read up on the company, but do not go crazy. You do not need to know when it was founded, just who their typical clientele are and what they do.
The Interview
Do not lie. Lying will always get you in trouble eventually. Beyond telling the truth, try the following:
Attire: Wear a suit, even if you're applying to a job at Best Buy. A suit shows that you care about the job, that the interviewer commands your respect, and that you know how to present yourself. Wear a black or navy suit, black wingtip shoes, a matching black belt (belts always match the shoes), white shirt, and modest tie. Your shirt should have collar stays, and not button down. Button-down collars are too casual for an interview. Do not try to be creative with your tie, you can differentiate yourself in other ways.
Hygiene: Take a shower, comb your hair, take out your piercings, your haircut should have been within the last four weeks, remove jewelry, and clip your nails. There will be a lot of eye contact, smiling, and talking, so right before the interview put a drop of Visine into both eyes to make them "pop" and eat a breathmint. It can't hurt.
Posture: Sit up straight, do not slouch, fold your hands in your lap if you have nothing else to do with them, and keep your elbows off the table, if applicable. Slouching makes you look like Mr. Burns from The Simpsons. When you own a nuclear power plant, then you can slouch, but not in an interview.
Smile: Smiling shows that you are a pleasant person. Smiling also tends to put people in a good mood, including yourself. I would say, "Be personable," but you cannot teach personality and it would be lying to fake it. Faking it comes off as just that: fake. So smile and be yourself.
Handshake: Give a firm handshake without crushing your interviewer. In the U.S. the handshake is still critical for many interviewers, and can cost you a job. Every follow-up call I've made to an interviewer has noted that I had a great handshake, so don't discount it. This is pretty easy, just shake hands with a couple friends and use varying degrees of strength until they tell you what's firm, weirdo.
Resume: Have a half-dozen copies of your resume on hand. You'll need one for yourself to follow along, if necessary for the "laundry-list" interviews, you'll need one for the interviewer, and multiple interviewers if they decide to shuffle you along to superiors and coworkers.
Leave baggage at home: If someone cut you off on the way to the interview, you spilled coffee on yourself, or you girlfriend dumped you that morning, too bad. The interviewer is seeing you for the first time and will not forgive your situation. Even better: leverage your bad situation. For example, "I apologize for my appearance, but I was nervous on my way over and it spilled. At least I get to pick up a new paid of pants." This reaction shows you can let stress go, that you recognize an error in your appearance, and demonstrates an ability to deal with that stress in a positive manner. If they notice you are stressed at the interview, crack a joke. "My girlfriend and I broke up yesterday, so I'm a little off my game today. Don't worry, though, it was time to upgrade to a new model anyway."
Knowledge: Don't even worry about it. Your resume landed the interview, so they already know you're qualified. Just answer the questions they ask about your previous employment, apply what you have done to what you will do for their firm, and ask a few questions about grey areas in the job posting. I'm not going to coach you on answers to question like, "Why should we hire you?" or, "What about the position appeals to you." because the best and most honest answers generally occur at the interview. They know when you have studied.
Follow-up
The day after the interview mail cards to everyone who interviewed you thanking them for the opportunity. Use proper titles and write them by hand (i.e. "Mr. Boswell, Thank you for taking the time to interview me for the position of x. I look forward to working at Widget Company. Sincerely, Joe Interviewee." This adds polish to the interview and can even correct mistakes. My first job called me after the received the card. They said, "We were waiting for the card so we could ask you if you'd like the job."
If you do not hear back in 10 days, call and ask them when you should expect to hear back about the position, even if they told you not to call. Ten days is enough time that you should be scheduled for a second-round interview or, at the least, notified.
Second round
Wear a different suit to the second round of interviews (navy or pinstripe). If you do not own a different suit, wear the same suit with a different understated tie. Add a pocket square. You can also wear a blue shirt and matching understated tie to a second-round interview. Do not try to differentiate yourself by appearance. A screaming orange tie, puppies, or Jackson Pollock will come off as unprofessional, even when interviewing at Moe's Tavern. Follow all of the above instructions again, answer their questions, and ask any that you missed the first time around. If you have no questions, I have found the following result to elicit smiles, "I asked all of my questions in the first round of interviews. I am here today to eliminate any doubts you have. I should really ask if you have any more questions."
Practice
Do not worry about getting turned down, even multiple times. It takes time to land a job. The average period of unemployment between jobs is three months when you are actively looking. Practice interviews with friends, your career department at your school, or just take interviews for a slew of jobs you are not interested in for practice. The more you interview, the easier it becomes to do them.
I will not delve much further into interview content (what is actually said during the interview). However, be ready to answer the question of what you are doing with your time while you are unemployed. Playing video games is not a response. Personally, I picked up a "bridge job." A bridge job is inconsequential employment you take between jobs to fill your time, learn new skills, and get some money. I went to Circuit City. It was fun, I worked with good people, met a lot of customers, and developed people skills. It was great, and it showed in my interviews.
I hope this helps some people. If you want more tips on typical questions that are asked and how to answer, you can find hundreds of better-written articles through Google searches. Best of luck, job hunters!