From Lean Development & the Predictability Paradox by Mary Poppendieck
A key technique for absorbing changes easily is to avoiding repetition like the plague. If you have to say the same thing in more than one place – either in design documents or code – then refactor the design to consolidate the capability into one place. One of the most effective ways to facilitate change is to localize every potential change in only one place.
From Extreme Programming Explained: Embrace Change (2nd Edition) by Kent Beck, Cynthia Andres:
If small steps are how to design, the next question is where in the system to improve the design. The simple heuristic I have found helpful is to eliminate duplication. If I have the same logic in two places, I work with the design to understand how I can have only one copy. Designs without duplication tend to be easy to change. You don't find yourself in the situation where you have to change the code in several places to add one feature.
IMO, if you couple minimizing repetition with writing code that is communicative, you should be in a fairly good place.