Polonius gives his son, Laertes, lots of advice prior to Laertes leaving Denmark to return to the European continent. He says,
Give thy thoughts no tongue,
Nor any unproportioned thought thy act (1.3.65–66).
In other words, Polonius says that Laertes should keep most of his thoughts to himself and not act on any immoderate or impulsive thoughts that he has. He also says to “Be … familiar, but by no means vulgar” (1.3.67), meaning Laertes should ingratiate himself with other people and get to know them, but he should not allow his behavior to range into the too-familiar or immodest.
Polonius also says to “Grapple [his friends] unto [his] soul with hoops of steel,” imploring Laertes to keep his true friends close and work to maintain those relationships (1.3.69).
Polonius also advises against “dull[ing] [his] palm with entertainment” or entering into quarrels with any frequency (1.3.70). However, Polonius says, should Laertes find himself in a quarrel, he should “Bear ‘t that th’ opposed may beware of [Laertes]”; in other words, Laertes should fight to win (1.3.73).
Polonius also tells Laertes to
Give every man thine ear, but few thy voice:
Take each man’s censure, but reserve thy judgment (1.3.74–75).
By this he means that Laertes should listen to everyone but speak his mind only to a few, and he should accept others’ criticism of him without offering his own criticism of them.
Polonius tells Laertes to dress well but not to overspend or try to be “fancy” (1.3.77). He also says, famously, “Neither a borrower or a lender be” (1.3.81). If one borrows, then they do not learn to live within their means. If one lends, they risk losing their money and their friend. Finally, Polonius says, “This above all: to thine own self be true,” perhaps his most famous piece of advice (1.3.84). In other words, Laertes should follow his own conscience and not be led astray by other people’s counsel or actions, and this will keep him on the straight and narrow.