This excerpt comes from Houghtaling's Handbook of Useful Information ©1889 but some of the truths are useful for today as well.
The Way for Business Men to Get Rich
The way to get credit is to be punctual in paying your bills. The way to preserve it is not to use it much. Settle often; have short accounts.
Trust no man's appearances–they are deceptive–perhaps assumed, for the purpose of obtaining credit. Beware of gaudy exterior. Rogues usually dress well. The rich are plain men. Trust him, if any, who carries but little on his back. Never trust him who flies into a passion on being dunned; make him pay quickly, if there be any virtue in the law.
Be well satisfied before you give a credit that those to whom you give it, are safe men to be trusted.
Sell your goods at a small advance, and never misrepresent them, for, those whom you once deceive will be aware of you the second time.
Deal uprightly with all men, and they will repose confidence in you, and soon become your permanent customers.
Beware of him who is an office seeker. men do not usually want an office when they have anything to do. A man's affairs are rather low when he seeks office for support.
Trust no stranger. Your goods are better than doubtful charges. What is character worth, if you make it cheap by crediting everybody.
Agree beforehand with every man about to do a job, and, if large, put it into writing. If any decline this, quit, or be cheated. Though you want a job ever so much, make all sure at the onset, and in a case at all doubtful, make sure of a guarantee. Be not afraid to ask it; the best test of responsibility; for, if offence be taken, you have escaped a loss.