Thinking more about this, I guess the Egyptians idea is relatively effective.
You just have to build monuments that will guarantee to impress, thousands of years from now.
I once read a Stephen Baxter short story where the last sequence for activating a viral re-writing of the DNA weapons hidden in the bodies of Inter galactic Aid-Workers were the various nursery rhymes and limericks that had been distributed ahead of the fleet in the social pantheon of the spacer-settlers.
The military could then distribute the fleet in enemy territories and ‘activate’ them once they had distributed into a particular volume of space where the final code words had been inserted.
