The Framers wrote the constitution knowing that needs and customs would change over time. What techniques did they're to ensure that the constitution would, in fact, be relevant for hundreds of years?

They were in a vision which of the future which is to encompass all necessary fundamentals, principles, issues, and foundation for their constituents. The constitution was broad enough to adapt to the changes and challenges that the people would face for the coming years. They have the foresight of the country which guided them to see the challenges that would come.