1. Do not sit there constantly smiling during the whole interview. The interviewer will see right through you and may see you as fake and phony. Of course there is the right time and reason to smile and a pleasant smile will always be appreciated but just don't sit there the entire time putting on a fake smile.
2. Make sure your clothing does not make you to warm during the interview, you may start sweating during the interview. Sweaty palms and a sweaty forehead will not impress the interviewer and you might be viewed as someone who gets way to nervous to easily which is not a good thing in an employer's eyes. Employers like people who can take a little pressure and unless you are being interviewed to be a personal trainer, don't sweat it.
3. Don't lie during an interview, a skilled interviewer will see right through your lies. There is no reason to not be proud of your past accomplishments but do not stretch them or over exaggerate them. Your interview might start out going very well and the interviewer may have in his/her mind to hire you but once your lie is detected you may greatly reduce your chance of getting hired.
4. Don't be petty and ask unnecessary questions. During the interview, don't ask questions such as "where is the restroom?" or "where is the closest soda machine?" These kinds of questions can wait until you are hired. Don't waste the interviewers time with such questions, chances are that they will give you a walk around tour anyways after hiring you and if not then just save these petty questions for your future co-workers.
5. Don't be a comedian during your interview. Jokes are great to break the ice when they are funny but it would probably be best to save them for family and friends. Trying to be funny will automatically show the interviewer how nervous you are. Besides that, you really don't know the interviewers personality and don't want to not get the job because you think that they are someone who appreciates jokes. Some interviewers might appreciate a joke but most will not and will see you as someone who doesn't take things serious.
6. Don't be a trash talker. Simply put, don't talk bad about your past employers. There are two sides to every story and you may flare up the interviewer's curiosity and tempt him/her to call your past employers. There is really no reasons to trash talk your past employers anyways.