I really think that memorization of the foundations and core of a programming language you expect to need, even infrequently, is the key to levelling up as a hacker.

Mind maps and/or spaced repetition techniques should both help tremendously as learning tools for that effort. Restricting what you "have to know" when faced with the need to know multiple programming languages feels like an important part of being successful.

I speculate there is a strong levelling up effect associated with immersive learning experiences, particularly with languages and idiomatic usage.