Be extraordinary in your excellence, if you like, but be ordinary in your display of it. - Baltasar Gracián

Don't be brittle as glass in dealing with people. And especially with friends. Some people crack easily, revealing their fragility. They fill up with offence and fill others with annoyance. They reveal a nature so petty and sensitive that it tolerates nothing, in jest or in earnest. The slightest thing offends them, so insults are never necessary. Those who have dealings with them have to tread carefully, always attending to their sensibilities and adjusting to their temperaments, since the slightest snub annoys them. They are completely self-centred...

Never compete with someone who has nothing to lose.

clxxxiii Do not hold your Views too firmly. Every fool is fully convinced, and every one fully persuaded is a fool: the more erroneous his judgment the more firmly he holds it.