There are two sides to an interview: you and the interviewee. Let's talk about you. Call in advance for an appointment if you plan a face-to-face interview.
Whether it's in person, by telephone or by email, you should: identify yourself clearly by name; point out that you are reporting for a publication, provide the name and give the URL if asked; and briefly explain what your story is about.
Be general, because the shape of most stories changes as you do more reporting.
Don't plunge into your interview unless the interviewee is really in a hurry. A minute or two of chat creates a less tense interview. Usually your least important questions come first.
A good carpenter goes to the job site with a full tool box. Make sure you have pencils (or pens), a notebook or pad of paper, and, if necessary, a tape recorder.
About your tools: Pens are more reliable than pencils. They don't break. A stenographer's notebook is usually easy to handle, but an 8 1/2 x 11-inch pad is sometimes preferred. Make sure your tape recorder has batteries and carry an extra tape.
Get the interviewee's permission before taping. In some places it is against the law to tape record what someone else says without explicit permission, whether it's on the telephone or in person. Some laws are national, others are by region or, as is the case in the U.S., by state. By answering email questions--assuming you made it clear they were in connection with a story for publication--the interviewee is in fact giving permission. Tape recording enables you to get perfectly-accurate quotes, but the process is often cumbersome and time-consuming. And guess what? Machines don't always work! Thus, it's best to take notes, even when taping.