dws's: "...manage commitments and expectations carefully"
footpad's: "...make sure you both agree you know what "Done" looks like"
trs80's:  "Requirements, Requirements, Requirements" , and, "Manage Expectations"

It is very easy to assume you understand what they said, or assume that they said what they meant. Likewise it's often ridiculously easy to assume they understood what you said, and wrote, and agreed to.

Work Very Diligently and Obviously to be sure you've each duplicated the other - throughout the project, not just at the beginning.

And do everything possible to document all agreements, changes, conversations, followup confirmations, etc. You will all be better off for it. They'll more likely get what they wanted and know it (even if you 'adjusted' their wants during the original scoping conversations), and you'll know that you delivered what you promised.