I love reading articles on things you should do and things you should ask in interviews.  Most of these items are pretty obvious but jobseeker still mess them up.  The item that bothers me the most is when a jobseeker is unprepared for the interview. 

Just remember that your goal in the interview process is to have this company make you an offer of employment.  So always remember that you are auditioning for the job. 

You should avoid these things at all costs:

1. Show up unprepared: You have time to prepare for an interview and you need to make the most of it.  If you fail to then what message are you sending to the interviewer.  Basically, you are saying that you are unable or unwilling to learn about this job.  Or you may be saying that it is not important for you to spend the time to learn about the company and your potential job.
2. Pay little attention to your appearance: You need to look like a professional regardless of the job and company you are applying for subject to the caveat that some companies have their independent cultures.  So whether it is business casual or business formal, you need to look neat and professional and carry yourself as such.
3. Say little or too much.  If you do not say anything then you are not providing the interviewer with any information to determine if you fit the position which means that they will conclude that you do not fit.  If you say too much, you will turn off the interviewer.  Also avoid going into too much personal detail like you went drinking las night.
4. Lie: Don’t lie about education, qualifications, past employment or a prison record. Most companies verify your background and in all likelihood they will find out.  If a company wants to hire you and finds that you lied, you may end up being fired.
5. Trash a former employer or fellow employees: Anytime you complain about a current or prior employer, employee or boss, it will be seen as sour grapes and that if you talk about others then you will end up complaining about your new company.
6. Too Casual.  Try not to be too familiar with your interviewers by calling them a nickname or inviting them out to a drink.
7. Don't Show Nervousness by fidgeting or biting your nails.  Employers are looking for confidence, especially if you’re expected to meet with clients or give presentations.
8. Be disrespectful: Even if you don’t agree, it’s best to hold your tongue. No business wants to hire someone who is disrespectful to others.   In fact, I had an interview in which the employer told me that I did not have a sense of humor.
9. Drink alcohol.  If you’re at a lunch interview, refrain from drinking.  Some companies test their potential employees by offering drinks to see if they have a drinking problem. 
10. Show up late: If you are late for an interview, it tells this employer that you think that you are either more important than they are or that you do not take this process seriously.  If you are going to be late, call them once you know. 
11. Ask no questions: There will come a time when an interviewer will ask you whether you have any questions.  Come prepared with 4 or 5 questions to ask.  By doing so, you give the impression that you care about this job opportunity.
12. Being rude or trying to make dates with employees.  How your interact with your potential other employees send a strong signal of the type of employee you will be.  Also these other employees have a great influence on the opinion on the interviewer.
13. Fail to follow up: Always follow up on the interview within a few days. It will show the employer you want the job, and also that you practice good business sense.  This includes sending a thank you note.