Being exposed to Debian's policies on having sources for everything, including non-software, helped.

Sometimes it's not easy: e.g. when I need a 2d shape in OpenSCAD I usually prepare it in inkscape as SVG, but then the conversion from an editable shape to a DXF involves a few manual steps, so I usually include both in the repo. Luckily this particular case will be solved with support for SVG in OpenSCAD itself, but I can think a few other examples.