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"Don't eat meat on Fridays."
To some people this old rule seems silly, but that's because people today aren't as sophisticated as people used to be.
Suppose it's ancient times, and you are the minister in charge of influencing people towards honorable behavior and away from devolution back to brutal life forms. One of the attributes that the people will need, in order to have good habits, is some degree of self-control, or self discipline. But, you can't order them to jog around the Sphinx every Tuesday, because it's too hard. They won't do it. You can't tell them to brush their teeth after every meal, or quit smoking, or wear some special kind of hat. They won't.
But the people need some very simple ritual to regularly follow, just so they don't slouch into complete liquidity. It's just an exercise, and everyone needs exercise. But, this exercise should be the least possible effort and the most possible perceived gain.
How about replacing meat with fish on Fridays? Say that it's in honor of the Friday crucifixion of Christ.
If you come up with something that feels like a very minor inconvenience, but has an aspect of ceremony and festivity around it, people will be more accepting of it. In fact, the Friday Fish Fry is one of the most knock-out delicious meals you can get -- if it's good, which would be in the northeast of America and maybe a few other places.
And, now people have at least one tiny touchstone with the practice of doing something slightly inconvenient every week. Like the person who jogs once a week, or pushes the lawnmower himself, these people will be somewhat acquainted with discipline, in some tiny way, which is way better than no way.
And did I mention? Self-discipline is by far the most important success skill that you can have.
Nothing else can accomplish what self-discipline can accomplish.