You can find a series of interesting tips on conducting different types of interview over at Wannabehacks, covering:
One of the tips I strongly support is not being afraid to hold a silence for a few seconds once you've asked a question. The interviewee may well fill that pause with an extra comment - maybe something they shouldn't really have said.
Listen and observe
Another tip I'd add is that good interviewing is about listening and observing.
Listen to what the interviewee is telling you and ask supplementary questions based on their answer. You'll be able to examine the issue in more detail, which means you'll probably get more useful quotes and information.
Look at - and/or listen to - how they answer too. Body language and speed and tone of speech can also give you a few clues as to whether you've hit on an interesting and/or sensitive topic.