Be friendly and confident, but remember that interviews are a means for reporters to conduct their business. Beware of the reporter who remains silent, or encourages you to ramble or dilute your original message. It is human nature to want to fill those lulls with conversation. If you don't want something to appear in print, don't say it. Keep it simple.
Nothing ruins an interview faster than long, complex explanations.
If you want your message conveyed, be sure to say it simply. Reporters are looking for quotes. Practice answering questions in 20 seconds or less. Chances are, the reporters will use the first decent 20-second comment and skip much of the rest. Tell the truth.