So the blessed day has come, you have your interview. We have already talked about how to prep before your interview day - now some last minute fine tuning and then what to do when you are in the midst of your discussions.
Ramping up or down:
Ramp up: If it is a morning interview, make sure you get a good night's sleep. If you are currently not employed, ramp up with some last minute study or review of notes/job/resume, read the Wall Street journal or an appropriate publication at breakfast while you are dressed for the interview. Get your "Business Head" on. If you are a "caffeinated" person, make sure you have the right amount, but not too amped up.

Ramp down: If you are working and coming from your job - if it is possible, put some space between any stressful work situations and the interview. Maybe grab a quick snack or lunch and do some prep, take a walk, light workout, yoga. No phone calls or constantly checking your PDA.

Just before you get there:

• Look in the mirror to make sure you look sharp.
• If you ate, brush your teeth or breath mints, etc.
• SHUT OFF YOUR CELL!!
• Use a restroom if available or ask the receptionist at the interview to use BEFORE the interview starts. It's OK to ask between interviews for a bathroom break, but you always want to be comfortable during.

Greet each person you meet cordially (especially the receptionist, they have more influence than CEO's in some companies' hiring!) - try not to be over zealous and match the energy of the person you are speaking with. Always stand to greet someone and wait for them to extend their hand to shake. Sometimes women will not do a handshake or there is something that prevents people from shaking with their right hand, so follow their lead.
If offered something to drink, it is perfectly OK to accept something simple like coffee, tea, water. Don't ask for soda or a half caff latte w/whipped cream and a cherry on top!
If there is food offered (don't ask for food) and you are hungry, take a small portion of something not messy and eat only before or between interviews.
During the interview, obviously sit in a seat that you are instructed to sit in. If you have any choice, try not to sit across a desk or table from interviewer, but at a 45 degree angle. This creates a more even "playing field" between you and the interviewer, that it is an information exchange vs. an interrogation. See the diagram:

When having conversations with various interviewers, make sure you are cognizant of their role and speak in a manner that they can relate to. Don't get in the weeds with tech speak to HR or project managers.
Make your answers concise and to the point. Don't say one more word than necessary, read the body language of the interviewer to gauge when they feel they have gotten their answer and gracefully end it there. If they start nodding their head and seem like they want to speak, that is a cue that you should stop.
Keep your answers positive and moving toward the goal of winning them over. Don't get into the business of knocking past employers, companies, etc. Maintain eye contact as much as possible during your interview especially when the interviewer is speaking to show that you are engaged. Take copious notes so you can follow up on something instead of messing with the conversation's flow. Like I said before, Always ask questions.
Maintain a feeling of relaxed confidence. Whether you have a job or not, interview like you have one and that this would be a new opportunity rather than a desperate lifeline. Don't ever be nonchalant or arrogant about the opportunity - show genuine enthusiasm, but make them feel they need to sell you on the opportunity as much as you need to sell them.
If you are in an interview that seems not to be going well due to the interviewer being adversarial or just in a plain bad mood, my friend "M" brought up a great idea. If this happens, he always tries to ask them about themselves and how long they they have been there, why they like the company, etc. Let them reveal any personal details or interests. This can be quite disarming and can go a long way to turning around this dynamic. People love to talk about themselves and most times can't do it in a grouchy manner.
When it is over, if you are staying put, stand and thank them for their time. Get a business card or their email address at a minimum.
Usually when all interviews are over - the HR person will come in to wrap up. Cite some instances that you had written down about what you liked about the job/company in your conversations. Ask what the next steps would be: How many people are interviewing? When do you expect to make a decision? When is it OK to follow up with you about status if I haven't heard from you yet? It is OK to ask if they have any feeling on the general feedback from your interviews. Most times they won't have any yet or won't divulge, so don't press the issue, just keep it in terms of "was it generally positive?" Collect any interviewer contact info from HR/Recruiter that you neglected to get.
When you leave, ALWAYS thank the receptionist for their assistance or at least say goodbye.
Send thank you notes within 48 hours of your interview. An email is perfectly acceptable. Again cite some key points of your specific conversation with each interviewer to use as evidence that you would be a great fit. Like the cover letter, make it brief with just a couple bullet points.
I know we have just skimmed the surface of interviewing, but I think this will provide a good general structure. If you have more specific questions, please contact me.
Next, we will discuss reference check and who to provide for them. Not your Mom! :-)