The pre-interview stuff:

1. Do your homework

This is almost always the first suggestion on every list, and I think it's pretty self-explanatory.

2. Know what your goal is

Brave New Traveler's Sara Stuteville says this:

Know what questions you’re going to ask and why you’re going to ask them.

Heading to an interview with a sense of what you want to get out of it (a colorful re-enactment of an event, an on-the-record opinion on the issue you’re covering, general background, etc.) is critical to conducting a successful interview.

You should already be thinking about what you want your piece to look like and what you need from this interview to get your article closer to that end result. link...

To this comment, I'd add that it doesn't hurt to share the angle you're exploring with your subject. Saying something like, "I read in your questionnaire that you think X, and I'd like to talk about that with you because it think it's a really interesting story," isn't weird, it's honest, and it helps the subject know what you think (about them) is interesting.

3. Pick a good spot for the interview

Sarah also offers this tidbit:

Avoid Starbucks! It’s often easiest to suggest a centrally located corporate coffee shop but if there is any way you can interview in a place that has some relevance to the story or your subject you’ll have much greater success.

Not only because you’ll gain a further sense of context, people are often more comfortable (and open) when they’re in a familiar place or what feels like “their territory.”

Ask to meet at your subject’s house, work, or the location of an incident relevant to the story. Even meeting at the interviewee’s favorite restaurant is more interesting than a Starbucks. link...

4. Have a list of questions

Even if it's just a list of topics to cover, plan to have a list of discussion items long enough to be able to pick and choose as you interview. That way you won't ever get stuck looking for the next question to ask. Pacifica radio's Nathan Moore suggests this when you're planning your questions:

The best questions have four basic qualities. They are open-ended, simple, neutral, and bland.

They’re simple but challenging questions. “Why?” “What did you say?” “What happened next?” “What do you mean?” “What makes you say that?” “So what?” “How do you know that?” “What’s the next step?” These are not the questions of polite conversation, so you may feel uncomfortable asking them at first. But these are the questions that elicit colorful, complex, contextualized, opinionated responses. link...