[consulting] Contract protection clause

[Bob Morse]

We have a client who is concerned with protecting his new Drupal website we will be developing. He is asking for a clause in the contract that somehow states we will not turn around and sell his site with a new design slapped on top to a competitor. I'm not sure how to separate out, ahead of time, what would be very common elements and what would be unique based on the internal processes of a competing business. 

How might I write something that assures the client we won't resell the unique aspects of his web application without also writing something that prevents us from making a website for another company that uses many of the same elements but with the details being unique to that company's internal process and interactions with clients?