How to ensure the design stays flexible is well documented. All the design principles, SOLID, Clean Architecture, etc. have been created for it. What is important is to maintain balance. No book teaches that. Take these design principles to extreme and you'll never launch. Do too less and you'll see problems in a year. A good designer anticipates things that will really change, sort of predict the future well. The usual suspects are the technology layer, because tech changes very fast. Definitely keep those flexible. Take judicious call in other layers based on market research, your experience, and your vision. Also, write tests because tests give you confidence to refactor or even redesign easily when the need arises.

Adopt modular design with clear interfaces. Follow SOLID principles and Separation of Concerns. Utilize design patterns like Strategy, Observer, or Factory. Build API-driven systems for easy integration. Implement CI/CD pipelines for faster, safer rollouts. Use dependency injection for interchangeable components. Leverage feature toggles for phased rollouts. Opt for scalable architectures (microservices/serverless). Prioritise automated testing for robust flexibility. Regularly refactor and document to keep the code clean.

In my view, a software can be divided into two parts. First part is the business logic that the team owns. Second part is everything that team does not own (frameworks, libraries, databases, cloud services, infrastructure in which its deployed, etc.). Design the software such that your business logic is decoupled from the second part wherever possible. One design principle that would help to achieve this is dependency inversion principle.

Ensuring software flexibility isn't just about technical strategies, it's about promoting adaptability within your team. While modular design and patterns matter, understanding the 'why' behind changes is key. Regularly assess market trends, user feedback, and emerging tech to anticipate future needs. The most adaptable software comes from a team that embraces change in the tech landscape. To cultivate a forward-thinking approach, execute easy configuration options for adjustments without major code changes. Adopt an iterative design strategy to evolve with changing needs. Remember, flexibility isn't about the most advanced architecture, but about creating a system that can smoothly accommodate new requirements and tech as they emerge.

Flexibility in software design means building not just for today but for the unknown demands of tomorrow. Beyond technical strategies, it requires a mindset shift. Modular architecture and design patterns like MVC lay a solid foundation, but true adaptability comes from fostering collaboration and feedback. Encourage diverse team input to anticipate varied needs. Prioritize clear communication, including well-documented code, to make transitions seamless. Finally, embrace change as an asset—your software should evolve with its users, reflecting their growth and resilience.