I often repeat this same advice, because it's what I think it most effective:
Come up with a detailed plan for your future... the next 5 to 10 years. Tell them the details of your creative plan and how this school's program and particular professors are crucial parts of the plan as you envision it.
This is not insincere, not if you coe up with a (tentative) plan and really go into detail about it. Most people have trouble doing this. It won't be easy, but you need to make a clear plan for the future. They will be able to know, by reading what you write, how much thought you have put into it.
So... stay up all night and make a great plan!