All or Nothing
I have worked on several projects where the customer insisted that it was "all or nothing"—they needed all the features, on the exact date, at high quality, or the project simply was not worth doing. With no lever to push, the success of the project relied entirely upon the feasibility of hitting the date.
In all of these cases, when push came to shove, the customer found functionality to cut. When they made the choice earlier in the project, they made the choice consciously, and they felt in control of the choice, customers were much more likely to view the project as a success, and the doomed-ness evaporated.
Suppose you want the customer to make the choice early and she keeps insisting that it is all or nothing. What can you do?
First, ask what features are the most important—the key features you need to deliver to test early.
Second, put it this way: "I want to get everything done on time. We may be able to. I may not need to even ask this, but if you had to pick just one feature to cut in order to make the deadline, what would it be ?"
Once the customer identifies that non-essential feature, ask for just one more. Then one more.
Pretty soon, without even meaning to do it, the customer has prioritized the features, allowing you to work on the key features first, and, more importantly, break the project into phases.
Sometimes, the pressure is turned up so high that the difference between reasonable management and unreasonable management is hard to tell; in that case, I have room for one additional technique.