Stuff that make LaTeX an actually pleasurable experience, but that I have to look up every single time:

Compilers

if then
Greek + Latin characters in the same text XeLaTeX
Springer Nature template pdfLaTeX

Macros

Example macro with two arguments that creates a clean version of an unwieldy URL whilst retaining all the https://s and encoded mess in its clickable version:

\newcommand{\cleanurl}[2]{\href{#1}{\nolinkurl{#2}}}
% usage: \cleanurl{https://www.unwieldy.nope}{unwieldy.nope}

Resizing stuff to text/column width

Essentially

\resizebox{\textwidth}{!}{whatever}
% replace \textwidth with \columnwidth as needed

but for tables, the box should be around the tabular and not wrap the entire table environment.