As part of my job I'm responsible for recruiting and interviewing potential new hires. I've been doing it for several years now, and I've seen quite a few bad ones. Now I'm not talking about people who are nervous or have a limp handshake. If you are going to be interviewing people, you have to learn to look past nervousness. What I'm talking about is conscious actions on the part of the job seeker that are an automatic ticket to show them the door.
Embellishing Your Resume
(Don't Do It)
Probably the worst thing you can do in an interview is lie. Whether you lie on your resume or to the interviewer's face, it's usually quite obvious. Not quite as bad as outright lying, but still a capital offense, is embellishing the facts.
Everything on your resume is fair game for me to ask you about. Everything. So if you're not ready or able to talk about something it shouldn't make it into the resume. For example, if you're looking for a web developer position but don't know more about HTML than what the acronym stands for, don't put HTML on your resume. Sounds extreme, but I've seen it done.
If you have some experience in a particular skill, make it clear how much experience you have. Don't imply you're more proficient with HTML than you are, if you are an amateur, put it in the hobby section. If you are an expert at it, that's great, just be prepared to back up what you put down.
The primary goal of your resume is to get an interview. After the interview I should be more impressed with you than before. Overstating anything on your resume may get you more interviews, but once you meet in person it will end up costing you a possible job.
I would rather hire someone who is modest about their abilities but willing to learn than someone who is cocky and thinks they're the best thing to happen since sliced bread.
The company I work for is very casual, even in the interview process. Despite this however, you should at least make an attempt to look professional. T-shirts with your favorite band on them are definitely not acceptable. T-shirts that read "Whore Island Drinking Team" are especially not OK. Laugh if you will, but I've seen it.
Business casual is one thing and usually OK, looking like you put no effort into the interview is a totally different issue. Even if the interview attire is casual, step it up a notch and try to impress me. Appearances are important, some might even argue they're just as important as what's in your resume.
Making Up Jobs
(Also known as lying)
One student we interviewed put "Asinine Engineering" on his resume just to make it look like he had more experience. We didn't even bother calling him for an interview, despite the fact that he was otherwise a good candidate.
Bad Body Odor Or Breath
Take A Shower And Brush Your Teeth
This one should go without saying, but if you smell bad I'm probably not going to take you seriously. I wish I didn't have to say this over and over, but APPEARANCES DO MATTER! I'm not going to hire you if you come in every day smelling like you haven't showered or washed your clothes in the last month.
Focusing On The Negative
(Be Positive Instead)
I had one interview where the candidate kept voicing his concern that he would not be paid well and the benefits didn't sounds too good. Despite the fact that you should not ask about salary or benefits unless the interviewer broaches the topic first, you definitely shouldn't insinuate that you're worth more than they will pay you.
Don't Talk About Money
Unless you're applying for a finance related job, don't talk about money. I talked to one guy at a career fair who kept telling me he was only interested in money and didn't care much about what the job was. Needless to say I never brought him in for an interview.
Interviewing is not easy, but some common sense will put you ahead of the pack. Be honest and straight forward, don't repulse the interviewer, and just enjoy the interview. Remember you're talking to a person who was probably in your position not too long ago.
One more thing, be prepared to ask questions about more than just your job duties. Ask the interviewer why they like coming to work every day or about what their favorite project was. If they feel you're genuinely interested, they will be more interested in you as well.