Successful radio interviews basically rely heavily on good execution of operation of recording equipment and sensible questioning.
However, to go into more detail...
Before you go to an interview, make sure you know what you're doing and are well acquainted with the controls of the recording equipment you will be using. Check the equipment is fully charged and in full working order BEFORE you leave. When you start your interview, make sure it's actually recording and the sound quality is of a publishable standard.
Unfortunately, mics can be temperamental and sensitive. Don't move about more than necessary and don't make any unnecessary noise as this can affect the sound quality. Make sure it is plugged into the correct socket and the sound is working correctly. Hold it about six inches from the mouth and speak across it rather than into it.
Don't worry about switching the machine on and off or pressing the pause button during the interview; it only makes editing harder and is more likely to cause mistakes. When you've finished recording, hold the mic still for a couple of seconds which will make editing a little easier and leave a bit of ambiance at the end rather than it just being an abrupt stop; making the entire piece sound more natural.
Places to Record
Don't always go for a quiet place, actuality sounds good and makes the piece seem more real. Natural sounds add texture to your piece. Try to avoid rooms with an obvious echo unless you want it; i.e. basketball stadium or church hall. If this can't be avoided, stand next to curtains or something similar to drown out the echo. It's important to be aware of sound quality and odd sounds; boilers, air conditioners or fluorescent light strips as this may ruin an entire piece. If in public, keep away from heavy traffic unless its directly relevant to your story. When outside, turn your back to the wind; recording outside can be problematic on a windy day. If you have to record in a really noisy place, hold the mic closer to your mouth so your voice will be more prominent over the background noise.
The best interviews are those that sound like unscripted chats. Don't write down a long list of questions, just jot down a few questions and key ideas. Listen to the answers, develop his/her answers if something they mention interests you by asking supplementary questions. Questions should be simple and straightforward but avoid making them closed questions with yes or no answers. Most importantly, do your research; know who you're interviewing and the best questions to ask them. Try not to be overly deferential to your interviewees, always be polite and in control. When they're answering, nod silently to encourage your interviewee but don't speak as this will affect your audio.