When writing complex business logic, it is critically important to maintain clean code though the judicious applications of Test Driven Development and Domain Driven Design. However, even these powerful techniques fall short of solving the problem at the heart of building complex software: building what the customer actually wants.
Domain Specific Languages (DSLs) allow us to capture complex business requirements in code written in the language of the customer. Once an ubiquitous language between you and your customer is defined and implemented as a DSL, the code can quite literally be given back to the customer to edit and refine. This is not a theory, or a myth. I have done this under real-world constraints and deadlines, and you can as well.