Modularity. Build platform agnostic platforms. The more your code is modular and composed of pieces you can swap out, the less dependent you are on a particular device, UI, templating language, API, backend language, storage model, database, platform. Modularity increases the chance that at least some of it can and will be re-used, improved, which you can then reincorporate. AKA building-blocks. AKA "small pieces loosely joined".
https://indieweb.org/modularity