Being interviewed is giving a performance. For most of us, it's not something that we are accustomed to and the very thought can make us catatonic. If you know your stuff, the uneasiness usually dissipates as soon as the first words are out and the answers start to flow.
Before the interview, determine what you want to say. Prepare five main points to work into every interview. Also have three to four subpoints under each of the five main categories, because you never know how long an interview will run. The second step is to strengthen those points with anecdotes, jokes, and statistics so that they are more memorable and entertaining. The third step is to listen to the interviewer's questions carefully and take control of the interview. Always be prepared for the unexpected, and get help from media coaches. Lastly, appear natural, spontaneous and unrehearsed, which takes lots of practice.
That's the formula for success in any interview situation!