Brass Rule: Lord Chesterfield's counsel to his son: "If you want to be pleased, then please!".
This is the perfect way to find friends and influence people, especially when combined with a fine disregard for the truth, and allows anyone sufficiently intelligent to "if in Rome, do as the Romans do", and profit by it, and "if among cannibales, do as cannibals do", likewise.
It also much contributes to civil peace, good moods, and success in life. (A none too fine regard for moral values, apart from personal advantage, helps considerably.)
A related Brass Rule - so called because of its deceptive similarity to the Golden Rule - that indeed may serve as a kind of logical ground for it, and also for much more, is "if I would not do it, someone else would do it", which in formal logic amounts to "if I someone else would not do it, I would do it", which may greatly contribute to one's welfare, with this fine sort of practical moral rule. (Arms dealers find great financially mediated happiness this way. It has the great logical advantage of justifying anything one desires.)
Another remarkably effective Brass Rule is "My country (group, club, party, family, friends, church), right or wrong!". This also has the great virtue of justifying just any atrocity, quite morally also, according to one's fellows.
One basic principle is Ovid's Rule, that explains much here and elsewhere: Video meliora proboque; deteriora sequor = I see the better and approve it is better, but I do the worse. This is the common - "human-all-too-human" - way of the human heart, if the worse happens to be the safer, the more pleasant, the more rewarding, or the more popular.