Everyone has a different way of presenting themselves to the media whether your goal is to be authoritative or entertaining. The main thing to keep in mind is to present yourself as an expert on your subject, whether you think you are or not. To be an expert, you don't have to be the world's most knowlegeable person about something... you simply need to know what you're talking about. If you research and write about a subject, stay up to date with the latest news regarding your topic and present yourself in a knowledgable way, then that makes you an expert.

Bottom line: you have to believe that you're an expert before the media or your audience will, so have faith in what you know and present yourself with an authoritative voice.