Do always conduct yourself as if you are determined to get the job you are discussing. Never close the door on an opportunity until you are sure about it.
Don't answer cell phone calls during the interview, and do turn off (or set to silent ring) your cell phone and/or pager.
Do show what you can do for the company rather than what the company can do for you.
Don't inquire about salary, vacations, bonuses, retirement, or other benefits until after you've received an offer. Be prepared for a question about your salary requirements, but do try and delay salary talk until you have an offer.
Do ask intelligent questions about the job, company, or industry. Don't ever not ask any questions -- it shows a lack of interest.
Do close the interview by telling the interviewer(s) that you want the job and asking about the next step in the process. (Some experts even say you should close the interview by asking for the job.)
Do try and get business cards from each person you interviewed with -- or at least the correct spelling of their first and last names. And don't make assumptions about simple names -- was it Jon or John -- get the spelling.
Do immediately take down notes after the interview concludes so you don't forget crucial details.
Do write thank you letters within 24 hours to each person who interviewed you. (You can see some sample thank-you letters here.) And do know all the rules of following up after the interview.