It is important that you dress nicely, and clean yourself. Brush your hair and teeth, cut your fingernails, look like you care about the way you look.
Learn a little about the company to which you are applying. (The history of the company, the statistics, the surrounding area, its involvement in the community)
Ask yourself questions that you thing the employer will ask, and make answers that you will stick to. (Why are you applying here?, what do you like about our company?, What do you like/dislike about yourself? Etc.) Make up an answer that you like, and stick with it. When you give your answer, sound confident about what you are saying.
Don't fidget in your chair, sit still, taking deep, confident breaths. Act like you belong there, and keep focused on your main goal, to get a job.
Keep eye contact with the interviewer, but don't stare. Look away every now and then and don't create an uncomfortable situation, which could lead into an uncomfortable relationship between you and your boss.
Speak at a normal pace, in a normal tone of voice. Don't take over the conversation and seem to aggressive, but don't seem to passive.
If there is trash in the hallway leading to the office, pick it up. Once you get into the office, politely say “I found this trash in the hallway, is there a trash basket in here?” This will score you major points.
