10 ideas
Rule is classified by what it rests on: religion, resolve (prudent statecraft), or caprice (the ruler's desires). Rule grounded in religion or resolve can endure, while rule driven by caprice decays quickly. This lets an adviser diagnose how durable a regime is and how dangerous it is to serve.
The book likens a ruler who depends on untrustworthy supporters to a man riding a lion. Outsiders fear him, but he fears his mount more than they do. Supporters who make a ruler formidable to others can be his own greatest danger.
When your counsel succeeds, let the sovereign appear as its author and never claim credit. Rulers resent advisers who seem to outshine them, so self-effacement protects the adviser's position and keeps his advice welcome. The one who seeks recognition for a good idea loses the influence it could have bought.
Instead of attacking a ruler's mistakes head-on, the adviser affirms and strengthens the judgments he gets right. Built up that way, the ruler's own good judgment comes to expose his errors. The correction reaches the ruler through what he already does well.
If the ruler treats you as a brother, respond by treating him as a father. His warmth does not license you to treat him as an equal. Closeness calls for more respect, not less.
A sovereign who craves praise can be steered by flatterers, who distort his judgment and push out honest counselors. The ruler must train himself to distrust praise and must reward those who speak truth. A courtier who flatters excessively marks himself as untrustworthy.
The book gives different kinds of good treatment to four groups. A true friend receives your blood and wealth, and acquaintances receive assistance and company. Cheerfulness and kindness are extended to the public, and enemies receive justice and fairness.
Train yourself to listen as deliberately as you train yourself to speak. Do not interrupt, and when someone tells you something you already know, let him finish without revealing that you knew it. Good listening wins goodwill and hides the extent of your knowledge.
Do not show off learning or eloquence in gatherings, because display breeds envy and makes others eager to see you fall. Knowledge should be used when it is needed, not advertised. Modesty about your abilities is presented as a practical defense, not just a virtue.
Test a person's character, loyalty, and discretion before admitting him as a friend, because a bad friend is a lasting liability, especially at court. Once a friend is chosen and proven, keep him and overlook minor faults rather than discarding him lightly. Guarding secrets matters most, because a disclosed secret cannot be taken back.