Before you actually sit down and interview someone, it's important that you know exactly why you are speaking with this specific person and what you want to learn from this interview. Stay focused and don't waste your or the interviewee's time with questions they can't answer or aren't relevant to them or the subject.
Do your research. Before you begin to set up interviews, make sure you have a basic knowledge of your subject. If you are interviewing someone you don't know, try to find out as much as you can about their history beforehand. Learn what you can about their industry in general and their specific business or subject speciality.. Gather statistics, look at news reports and read articles that mention them, their business or the topic you're researching.
Have a list of questions. In other words, don't wing it! As you do your research and prepare for the interview, create a list of questions to ask. Remember to allow for some improvisation and adaptation based on what the responses are. Be sure you're asking the right questions of the right people (for example, asking your English teacher what it's like to be a car mechanic may not be as useful as asking a car mechanic), which may mean creating several lists depending on your research project.
Listen to the responses. Don't think about the next question while the interviewee is answering the previous one, listen to see if there's a follow-up question you may need/want to ask or if the question has actually been answered.