I wouldn’t dare defile Douglas Adam’s memory by not mentioning that you should keep a towel with you at all times, but my second contender is a surprisingly short three-parter:
- never lie.
- never tell the whole truth.
- never pass up a chance to use a real bathroom.
Both. It’s like the saying “Governing a big country is like cooking small fish.” (With the explanation that if you keep poking it, it’ll disintegrate) also taught me how to cook fish as well as realpolitik.
The fish advice was most useful.