Classical Wisdom for Everyday Use

It is an elegant and most desirable skill in conversation to escape cliche while maintaining simplicity, and conserve the wisdom of the commonplace in a better guise. Among the learned this is easily done with allusions to literature, and is not infelicitous even among the vulgar, so long as they are apt to be impressed. Yet where one's company possesses both the shapely mind of a good education, and an open character that neither marvels nor is haughty, the choicest adornments of an elevated exchange may be found in the words of the ancient writers. Every sentiment is weightier in Latin, nothing glimmers like a touch of Greek. One should therefore always have some phrases ready; I give you some examples.

Cliche: "Don't judge a book by its cover."
Classical Solution: "Nimium ne crede colori" (Vergil, Eclogues II.17; "trust not too much to color.")
Implications to bear in mind among the exceeding learned: The first half of the line is "O formose puer" ("O pretty boy!"); one may therefore deploy the line admonishingly with a subtle grin in the right company.

Common Phrase: "Child Prodigy"
Classical Solution: "Non sine dis animosus infans" (Horace, Odes III.4.20; "An inspired infant, and not without the patronage of heaven.")
Secondary Use: Horace is describing himself as a child, and one could therefore use the line ironically of the self-impressed, although this may lose some sting if the subject, like Horace, is at all worthy of the line.

Political Opinion: "We ought not to appease, Munich, Chamberlain, etc."
Classical Solution: "...χειροτονήσετε ὦ ἄνδρες ᾿Αθηναῖοι ἴνα μὴ μόνον ἐν τοῖς ψηφίσμασι καὶ ταῖς ἐπιστολαῖς πολεμῆτε Φιλίππῳ ἀλλὰ καὶ ἐν τοῖς ἔργοις." (Demosthenes, 1st Philippic, section 30; "Now vote, O men of Athens, so you may be at war with Philip not only in letters and resolutions, but in your actions as well.")
Problems of Employment: The sentence is elegant if not pithy, and long enough that quotation in the original is rarely to be advised. A more general allusion is preferable: "As Demosthenes said, you must fight tyrants with deeds as well as resolutions."