Ask HN: Most useful things you've learned that doesn't “expire”?

Much of what we learn will most likely only be relevant for a short period of time (ex. some new trendy Javascript framework) or not useful out of the immediate context (ex. internal libraries at a company). I've been looking for some useful things to learn more about that have/will have a more long term impact. For example, the popular "Learning how to Learn" course has been extremely useful for many people and teaches tips/tricks that they will use for the rest of their lives.