Modeling Principle: Models are not right or wrong; they are more or less useful. - Martin Fowler

When you find you have to add a feature to a program, and the program's code is not structured in a convenient way to add the feature, first refactor the program to make it easy to add the feature, then add the feature.

Refactoring is a disciplined technique for restructuring an existing body of code, altering its internal structure without changing its external behavior. Its heart is a series of small behavior preserving transformations. Each transformation (called a 'refactoring') does little, but a sequence of transformations can produce a significant restructuring. Since each refactoring is small, it's less likely to go wrong. The system is also kept fully working after each small refactoring, reducing the chances that a system can get seriously broken during the restructuring.

Refactoring (noun): a change made to the internal structure of software to make it easier to understand and cheaper to modify without changing the observable behavior of the software. To refactor (verb): to restructure software by applying a series of refactorings without changing the observable behavior of the software.