Arrive at the interview with enough time to fill out any application paperwork they might require. If you have a cell phone or a beeper this is a good time to make sure it is turned off. You may be judged just as much during your wait in the lobby as you will during the interview so make sure to handle yourself professionally, and treat everyone there the way you would treat the interviewer. Most employers consult their assistants before making job-hiring decisions and this might be your only contact with them.
When greeted by your interviewer make sure to shake hands and introduce yourself and smile. Most people make a decision about a person within the few moments after meeting them, so make those few seconds count. Be sure to thank the interviewer for their time. It's also a good idea to make some small talk before the interview begins. Just make sure to keep it positive!
The interviewer might have a set of questions ready for you or might base the interview off your resume. Either way be ready to answer all questions honestly while still hitting your key strengths. Don't be afraid to ask for time to think if the interviewer asks you a difficult question. Be sure to refrain from any nervous habits like fidgeting with your clothes or hair or tapping your foot.
Do not bad mouth past jobs or bosses, no hiring manager wants to hear about your negative past experiences in jobs. Always keep it positive. Don't ask about the salary until after they have offered you the job. If they ask about what salary you are looking for try to remain vague and tell them you want a "competitive" salary. If you must give figures, give a wide range.
When the interview is wrapping up make sure you get business cards from everyone at the interview. It will help jog your memory when sending out thank you cards. Make sure to thank the interviewer for their time and shake their hand. Don't be afraid to ask when a decision will be made so that you can schedule a time to call and check in. On your way out, remember the interview is not over until you leave the building, so be on your best behavior until you are out of the building.