A good rule of thumb is to always answer, "Okay. What should we drop to get this in by the deadline?" and/or "Okay. If we move the deadline to X, we can add that in."
Every change affects completion time. There's no such thing as a zero-time task. Forcing a project manager to realize that quality, deadline, or feature list will suffer every time they make a change will go a long way towards getting them to think right about scope creep.
