You generally have only 15 to 20 minutes in a job interview to prove that you are the right candidate for the position. This is why it's imperative that you don't do anything off-color during those precious minutes.
The first thing you need to be conscious of during an interview is your body language. It's OK to cross your arms in front of friends, but doing so while explaining why you should be hired will send a bad message to the employer. It says that you are nervous or, even worse, not interested. Posture is also very important. You don't want to slouch in the chair, but you also don't want to appear like a cardboard cut-out.
Keep the interview positive. Maybe you hated your last job and have nothing but bad things to say about it, but keep those feelings to yourself. Nonprofits want to hire someone who is positive, not someone who is always focusing on the negatives. If you are asked why you left your last position, say something along the lines of "I just felt it was time to move on to another job."
Eye contact is another important aspect of good etiquette, and it's also one that can be tricky to master. How do you maintain good contact with the interviewer without staring? As a rule of thumb, you should look into the person's eyes for no more than 4-5 seconds. Make sure to blink normally, and nod or shift your head during the conversation so you don't look like a statue.
The first and last thing you do in a job interview is shake hands with the hiring manager. The handshake is another difficult skill to get right, so practice it with people you know before the big day. It should be firm, but not so firm that you crush the person's hand. A great handshake can say a lot about you as a person, so make sure you make a good impression.
Acing these forms of job interview etiquette will not guarantee you the position, but it will improve your chances. Make sure to be conscious of the above rules the next time you are called in for an interview.