Comment by ninalanyon

Make sure that you have as thorough a specification of what the system is supposed to do as you can.

Then define the version control system and the build process, specifying the dependencies, and so on.

Think about any opaque blobs in the system and try to eliminate them so that you have plain text source code so that no tools are needed to read the code.

Make sure that the build process runs entirely locally and never fetches anything from outside.

Simplify everything, use only tools and languages that are well understood and supported.

The real problems are not strictly technical but social: how do you prevent loss of the code, the tools, the specification, how do you maintain the expertise needed to maintain it. How do you ensure that all those things that are obvious to you now are written down in all their gory detail so that your great grandchildren will not apply their new and different preconceived ideas to the system?

Document all this on paper as well as electronic storage, make sure that version ids are recorded on every page as well as being available to the user of the machine or program.

In the industry in which I worked for the last thirty years of my career it was not uncommon to have things come back for repair after fifty years use and to be able to consult the original drawings and bill of materials so that exact replacement parts could be made.