Make sure you know what the client really wants. You are working for them as a means to an end. Make sure you know what this end result is supposed to be.
Make sure you get their statement of this. A large scope document is often included, but it is important that you have an understanding of what is really wanted.
An example I have heard is: In building a palace, you might deliver great quality, great cost savings, and be ahead of schedule, but what the king really wants is something amazing to look at (more gold, more turrets etc).