Chose the camera angle that best suits the type of show you are conducting. If it is lighter fair like "The Tonight Show" feature the host and guest side by side with the camera pointed straight at them. Alternate cameras should provide close-ups for both the host and the guest.
More serious interviews should be conducted using a cross-over camera style with the two people sitting across from each other. A camera to the right and behind the guest will point at the interviewer while a camera to the left and behind the interviewer will point at the guest.
Prepare your guest and get them in the conversation mood with a pre-interview. This is essential in getting the most relaxed interview possible. Introduce yourself with a casual greeting and then proceed to ask "off the camera" questions to prepare for communication. Do not reveal any of the future questions; this will ensure that the subject reveals an honest reaction and answer when they are first asked.
Prepare a long list of questions to choose from so that you have enough information to cover. Keep the most important questions on a separate list and go ahead to the backup questions every so often. Be a good listener and come up with questions on the spot as well.
While interviewing someone, always make eye contact with the guest rather than the camera. It looks awkward when a subject is talking directly into the camera and the interview will look more professional and natural with the angled and focused look on the interviewer.
Have B-roll footage prepared to cut into during an interview. A static shot can become boring fast, so B-roll of the subject they are talking about can provide more interest for the viewers.