Create good relationships by planning ahead
As a journalist or researcher, you need not only to know how to get and write up an interview, but also how to create good relationships with your interviewees: you'll receive better information that way, and have a better chance of talking to people again if you need to.
If you're doing a pre-arranged interview, as opposed to an on-the-scene interview, you have the luxury of planning ahead. You can conduct interviews over the phone, by e-mail, or in person: the last option is usually preferable, but for busy or hard-to-reach subjects, the other two can also work well.
In any case, you should call ahead to ask for the interview and arrange a time to do it. Make these arrangements as far ahead of time as possible. (For e-mail interviews, let your subject know when your questions will be arriving, and when you'll need their responses.)