Martin Fowler lead a panel discussion at QCON2007 that included Dan North, Fred George, David Farley, Eric Doernenburg, and Ian Cartright. The main themes of the discussion were:
1. Do the simplest thing that can possibly work. Simple does not mean easy, quick or stupid. Simple means it is simple to use. Do the simple thing well before moving into the clever step.
2. Push decisions to last responsible moment. Always ask do I really need to make this decision now or can I put it off for later. Is a decision reversible? If it is then it is not a big deal. But if it is not, postpone it until the latest possible. Early up front design can lead to surprising results. Let the design come on its own with each iteration.
3. Domain Driven Design is natural home for OO. OO is about creating a software simulation of the problem we are trying to solve. The domain model should not know about the presentation layer or the data persistence layer. The domain model will change. Try to get a simulation as clean as possible without technical dependencies. Programmers use the same language as the business person. If the domain model is complex, design the simplest representation possible to get the job done and add to it as more requirements come along. Every time circumstances change and invalidate your model, fix the model. You should be able to articulate the architecture and be able to say this does not fit anymore.
4. Use encapsulation. Ensure classes keep secrets, use dependency injection.
5. Using Test Driven Development drives better design by making it more modular. You benefit from the solution as well as the testing. Having an automated unit tests and acceptance test enables change even in the database schema.
6. Mitigate risks by starting out on parallel path to test out different alternatives.
7. Have basic design beliefs like refactoring, design patterns, design principles, MVC, isolating db from model, focusing on model. Following these believes might not be the quickest way but it is the cleanest.
This presentation is available on InfoQ at http://www.infoq.com/presentations/modifiability-fowler