How Do You Approach Writing Scalable and Maintainable Code?

Writing code that works is one thing; writing code that scales and is maintainable over time is a whole different challenge. As projects grow, poorly structured code can lead to technical debt, making it harder for teams to add features or fix bugs. What are your go-to strategies or principles for keeping codebases clean, modular, and scalable? Do you follow specific design patterns, rely on robust testing, or enforce strict code reviews?