Without going too much into how they arise, I'd say make them systematic. Use linear layouts - clumpy, without being just thin strings. If you want a broad mass of roughness like the Himalayas or Alps, even therein you see linear groupings.

Variety works. I.e. not all continents = central lump of mountains surrounded by wide swaths of plains (sombrero form), nor only coastal. Run some out to sea, where sea level drowned some formerly-dry peaks, and have those taper off from bold peaks onshore to peninsulas along the axis of your range, to clumps of isles, to isolated islets, to (if you're showing underwater relief) banks and seafloor ridges. Vary your heights too. Even on a highly-symbolized hand-drawn style you can distinguish between this range that's all rounded and worn-down, and that one which is young and pointy.

Give your ranges some foothills, if the style permits.

Those kind of generalities?