Do The Simplest Thing That Could Possibly Work
DoSimpleThings enables design via two rules: First, implement a new capability in the simplest way you can think of that "could possibly work". Don't build a lot of amazing superstructure, don't do anything fancy, just put it in. Use an if statement, even. Make the code pass the Unit Tests for the new feature (and all features, as always). Second, and this is critical to the rule, refactor the system to be the simplest possible code including all the features it now has. Follow the rule of OnceAndOnlyOnce and the other code quality rules to make the system as clean as it can possibly be.