A friend consulting with me on an Data Warehouse development effort (Lee Laniear) once told me an excellent metaphor that explains the three types of software development projects.
You are like an archer with a quiver full of arrows. In the first type of project, your user places the target in the distance, you grab and load your straightest arrow from your quiver, you draw back the bowstring, take aim, and you give it your best shot. The objective, of course, is to hit the bullseye. You may hit or you may miss, and certainly the aim is more difficult with increasing distance to the target or with increasing crosswinds. But the main point is that you can see where you want to go.
In the second kind of project, you are still the archer, but the user runs back and forth fifty yards away carrying the target. The objective, of course, is to hit the bullseye (not the client). He may stop, run one way, then suddenly stop and run another. He may stand still for brief periods of time, and then without warning start running around again. You grab your straightest arrow from your quiver, you draw back the bowstring, and you aim where you think the target is going to be by the time your arrow reaches it. Your success depends to a great extent on your historical observations of how the user changes his mind. And your self-control to aim safely for the target and not allow your emotions to divert your attention toward more animated ends.
In the third kind of project, you grab your straightest arrow from your quiver, you draw back the bowstring, and then leaning backward, you shoot your arrow straight up into the air. The object is to get the user, carrying the target on his head, to position himself directly underneath the falling arrow.
Successful project managers have an ability to separate themselves from their immediate feelings about the folks providing them with work. They need to be able to understand a larger picture of how others view their world, have a sunny disposition, and cultivate a fair amount of self-effacing graceful humor.