Sit up straight in the chair or stand up and sound alert. Don't shout or whisper. Speak in normal tones over the telephone mouthpiece, not directly into it. Never use a speaker phone unless asked by the reporter to do so. Watch those vocalized pauses.
Silence sounds better and is effective for emphasizing a point.
Be sure you have a good, clear telephone line. This interview is a little different from radio. The print reporter is likely looking to move deeper into the subject matter than the radio reporter. In print, you have more of an opportunity to be specific on statistics and ideas that are difficult to convey on the air. Find out when and how the material is to be used so your responses will be appropriate to the target audience.