In each and every case, the first step is exactly the same. Reread the book you have written that will be the focus of the interview. You can't be expected to sound like an expert on this subject (and the interviewer and the public will expect you to be an expert ... after all, you wrote the book) unless you brush up on the facts. Nobody remembers everything they researched and wrote for any one book.
Take notes on major talking points related to the subject at hand (this is a chance to promote your work after all).
If the interviewer asks for a list of questions (this does not often happen), think hard and generate good questions that will get to the heart of your book, interest listeners/viewers enough they want to buy the book (always an interview goal to be remembered), and be the sorts of questions that will not stump you. Take advantage of the situation.
Be friendly with the interviewer and as helpful as possible. Like working with an editor, this is an opportunity to build bridges for future interviews. If an interviewer finds you handy for one topic, he or she is more likely to come back to you again as a reliable source if you've written on more than one topic.
Never, ever provide one word answers to any question. That is lousy interview technique and makes for a crummy interview nobody wants to hear, see, or read.
Always sound/appear relaxed, no matter how you feel inside. People like lively people to watch and listen to. If you are relaxed, you will remember your favorite stories, be engaged (people can tell when you're nervous and when you're relaxed. If you're nervous, they focus on that and not on what you are saying) in your topic, and provide material the interviewer's editor will want to use.
For radio or TV, speak briskly and use your public speaking voice. Briskly spoken words lends you an air of authority. Speaking up ensures you will be heard by all your listeners/viewers and that you make your editor's life easier.
Finally, to appear/sound relaxed, you need to remember one thing: you know more about this topic than any of your listeners, readers, and viewers. Nobody else has written your book, used your resources, or lived your life experiences. That is why no two books are alike and why you are THE expert on this particular book.
Relax, prepare, you're going to be great!