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

Some techniques the framers used to ensure that the constitution would be relevant for hundreds of years were the following:

They established a Constitution with general laws and principles that not only addressed the specific challenges that the young nation was facing at the moment (like unusual and cruel punishment), but also those to come. Some of those principles the Constitution is based upon are federalism (reflected in the fact that federal law takes precedence over state law), the Separation of Power and the protection of individual liberties. They granted the Judicial Branch the authority to evaluate and interpret those generals statutes from the Constitution in order to adapt it to an ever-changing society. The controversial 2nd Amendment, for example, has been interpreted as one that protects people's right to have firearms for self-denfese in the home, while prohibiting "the possession of firearms by felons and the mentally ill" and "the carrying of dangerous and unusual weapons." They outlined the procedure in which the Constitution can be changed or amended so that Americans can eliminate or add statutes that are relevant to their time.

They wrote stuff that wasn't specific to their time period that would be relevant in the future.