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.
Money is not the only medium of exchange.
Found the Ferengi.
Rule of acquisition #39: A favor is the most powerful type of debt.