Steve Krug is an acknowledged expert on usability testing. Over the years he has come up with a very usable(!) technique for picking up usability problems on websites. And he’s made it so easy that everyone (with a penchant for it) can do it. I bought his book and it’s great. As an accompaniment to the book he provides this user testing video to help get the idea. Watch it, it’s only 25 minutes:
Design for a better world