Stuff that make LaTeX definitely worth it, but that I have to look up every single time:

Compilers

if then
Multiple scripts in the same document 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
% prepend 0.N to \xxxwidth if the content should be resized to a fraction of the given width

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

Adding a full-page segment to a two-column paper

\onecolumn

Adjusting margins

\usepackage[margin=Xin]{geometry}

Glossed linguistic examples

Add

\usepackage{covington}

and optionally

\setglossoptions{
    fspreamble=\scshape\small, % style of the pre-gloss text
    fsi=\itshape,              % style of the main text
    fsii=\normalfont}          % style of the gloss

(there is also a bunch of other options, described in the awfully long incredibly thorough covington package docs; these are the ones I most commonly use)

Use as:

\begin{example}
    \label{utbildning}
    \digloss[preamble=Swedish]
        {En bra mening behöver inte vara en bra exempelmening .}
        {a good sentence need.PRES not be a good example.sentence .}
        {A good sentence is not necessarily a good example sentence. }
\end{example}

which will render as

The example above rendered in a PDF

\triglosses are also possible.

Ge`ez and Latin script in the same document

\usepackage{polyglossia}
\usepackage{microtype, newunicodechar}
\usepackage[sf, bf, big]{titlesec}
\defaultfontfeatures{Scale=MatchUppercase}
\setmainfont{Abyssinica SIL}[Scale=1]
\setsansfont{Libertinus Sans}
\setmainlanguage{english}
\setotherlanguage{amharic}
\newunicodechar{፡}{፡\ }
\newunicodechar{።}{\@{።} }
\newunicodechar{፣}{፣ }
\newunicodechar{፤}{፤ }
\newunicodechar{፥}{፥ }
\newunicodechar{፦}{፦ }
\newunicodechar{፧}{\@{፧} }
\newunicodechar{፨}{\@{፨} }
\newunicodechar{፠}{\@{፠} }
\newfontfamily{\amharicfont}{Abyssinica SIL}[
  Script=Ethiopic,
  Ligatures=Common,
  WordSpace = {0.1,30.0,1.0}]