1.  Not doing your homework.  The number one biggest job interviewing  mistake is to fail to research the company for which you're interviewing.  Remember to always go to the company website (if they have one) and do a little digging around.  Find out the CEO's name, or any news that has come out of the company lately.  Failure to be prepared with knowledge of the company shows disinterest to the interviewer. 

2.  Being late for the interview.  Make sure you leave with plenty of time to get to the location, park, and find the office.  You might event want to drive the route the night before so you'll know where to park and how far the building is from the parking lot.  If you end up late to your interview, you've already lost the interview before stepping in the room.

3.  Dressing inappropriately for the interview.  Notice I used the word "inappropriately." That means you can be under-dressed as well as over-dressed.  For example, casual clothes are inappropriate for a corporate job.  A suit and tie can be inappropriate for a creative job or laid-back company.  For women, many companies even frown on open-toed shoes.  The more you know about the company, the more you know what appropriate and inappropriate clothing is to that company.

4.  Not asking questions.  When the interviewer asks you at the end of the interview if you have any questions for them, don't just smile and say "no."  Come prepared with some questions which show your interest in the company. 

5.  Criticizing your former boss or co-workers.  Above all, don't dish the dirt about your current or former employers or co-workers.  If you start criticizing former employers, the interviewer might think you might do the same regarding your employment with them someday.  If you have any disagreement with current or former employers, be graceful in how you handle that subject in your interview.