1. Don't Prepare
Not being able to answer the question "What do you know about this company?" might just end your quest for employment, at least with this employer. Background information including company history, locations, divisions, and a mission statement are available in an "About Us" section on most company web sites. Review it ahead of time, then print it out and read it over just before your interview to refresh your memory.
2. Dress Inappropriately
Dressing inappropriately can work both ways. You will certainly want to wear a suit if you are interviewing for professional position. When interviewing for a summer job at your local theme park or as a lifeguard, for example, dress accordingly in neat and casual attire. If you aren't sure what to wear, visit the organization and watch employees coming in and out of the office to see what they are wearing.
3. Poor Communication Skills
It's important to communicate well with everyone you meet in your search for employment. It is, however, most important to positively connect with the person who might hire you. Shake hands, make eye contact, exude confidence, engage the person you are speaking with, and you will let the interviewer know that you are an excellent candidate for this position - before you even answer an interview question.
4. Too Much Communication
Believe it or not, a recent candidate for employment, who, by the way, didn't get the job, didn't hesitate to answer his cell phone when it rang during an interview. Leave the phone behind or at least turn it off before you enter the building. Same goes for coffee, food and anything else other than you, your resume, your job application, and your list of references. They don't belong at an interview.
5. Talk Too Much
There is nothing much worse than interviewing someone who goes on and on and on... The interviewer really doesn't need to know your whole life story. Keep your answers succinct, to-the-point and focused and don't ramble - simply answer the question.
6. Don't Talk Enough
It's really hard to communicate with someone who answers a question with a word or two. I remember a couple of interviews where I felt like I was pulling teeth to get any answers from the candidate. It wasn't pleasant. So, even though you shouldn't talk too much, you do want to be responsive and fully answer the question as best you can.
7. Fuzzy Facts
Even if you have submitted a resume when you applied for the job, you may also be asked to fill out a job application. Make sure you know the information you will need to complete an application including dates of prior employment, graduation dates, and employer contact information.
8. Give the Wrong Answer
Make sure you listen to the question and take a moment to gather your thoughts before you respond. Like the following candidate, you'll knock yourself out of contention if you give the wrong answer.
The interviewer had completely described a sales and marketing position to the candidate. She emphasized that cold calling and prospecting were the most important skills and experiences needed for the position. The candidate responded to the question about what she did or didn't like to do in sales, with these words: "I hate to do cold calling and prospecting, and I'm not good at it." That response ensured that she wouldn't get the job!
9. Badmouthing Past Employers
Your last boss was an idiot? Everyone in the company was a jerk? You hated your job and couldn't wait to leave? Even if it's true don't say so. I cringed when I heard someone ranting and raving about the last company she worked for. That company happened to be our largest customer and, of course, I wasn't going to hire someone who felt that way about the company and everyone who worked there.
It's sometimes a smaller world than you think and you don't know who your interviewer might know, including that boss who is an idiot... You also don't want the interviewer to think that you might speak that way about his or her company if you leave on terms that aren't the best.
10. Forget to Follow Up
Afraid you didn't make the best impression? Are you sure that you aced the interviewed? Either way, be sure to follow up with a thank you note reiterating your interest in the position and the company.
Finally, even if you do flub the interview, don't take it to heart. I don't think there is anyone hasn't blown an interview or two. If it happens, look at it like it just wasn't meant to be, learn from your mistakes and move on to the next opportunity.