Back at NAB 2014, Dave Dugdale, who runs Learning DSLR Video, took some interview footage and b-roll he had shot previously, and sat down with editor Chris Fenwick for a three-hour editing session. The goal was to see what Chris could do with the footage having never seen it before. The video has some great tips for editing interviews down to their most powerful and interesting bits, and if you've never used Final Cut Pro X (or you want to know it better), Chris talks about how he uses the program effectively.
A few weeks later Dave put together a great 30 second commercial from footage he had shot, and shared some of the things he learned while making it:
While you can only do so much with footage that has already been shot, if you're the one shooting the video, there are some things you can do to improve your chances of having a usable interview in post. If it's the type of video where the audience won't hear the question being asked, it's usually a good idea to try to have the interviewee repeat the question in their answer. This way you'll get a full soundbite rather than a truncated answer that may not fit in very well without some context. It can also be helpful for the interviewee not to reference other answers they've given earlier in the interview, since it's possible you won't be using that other answer in the edit.
For more on both of these videos, head on over to Dave's site using the links below.