I already wrote about understanding the big picture, but once you “get it” what do you exactly do with it? There is one particular thing that saved my life many times already. Develop a good summary of your requirements and leverage it immediately to maximize impact. It never fails if done correctly, so it pains me when I see some “fluff” taking the most important “space” in your documentation. Which is unfortunately quite often. So here is what should you do and how.
However you call this on your project (Management summary, Vision, you name it) those few paragraphs or a single page are developed with single goal in mind: to explicitly agree with customer’s boss on which exact problem (or problems) is being solved and how. You don’t want to get into situation where everybody seems to expect from you to solve each related problem along the way! Keep it short and to the point. Be sure to say what is NOT being solved by the project. For example you can say “Reporting won’t be done as part of this project”.
Once you have high quality draft, go to the customer’s boss and promise her you won’t need her to read anything ever again if she helps you to agree on those several paragraphs (it works every single time). What you will get is the short statement developed in fact by the boss (this is crucial!) of the people you are going to work with. Thus nobody will manipulate you in another direction: you can simply say “It’s a great idea, but let’s check what your boss wanted us to do…”.
Simple, isn’t it? And it gets even better.
You or (even better) your project manager should read this word by word on a kick-off meeting. And explicitly say the magical sentence: “This project won’t solve all the problems you may have, but it WILL solve those particular problems: …”. The point is that at this particular moment your customer and probably even his boss are standing there right beside you and nodding their heads meaningfully!
What you get is magically agreed upon scope project-wide – almost out of the box 
