Continuous attention to technical excellence and good design enhances agility.
Be awesome. The best way to become awesome: always be learning. Take every opportunity to learn more about your craft. Read books and blogs. Watch video courses. Attend conferences. Discuss technical issues with your peers.
Learn the SOLID principles. Commit yourself to writing clean code. Study and follow the best practices of software engineering. Avoid writing brittle code.
Work with a plan. Application architecture isn’t just a fancy word. Developers who plot out an intelligent plan and follow it through have more success than those who blunder through problems blindly.
And most of all, try to minimize the creation of technical debt. Sometimes it is unavoidable; the world is not an orderly place. Whenever you can, you should pay down technical debt while delivering new business value. Maybe you can include some high-quality refactoring as part of the delivery of a new feature.