As Laertes is leaving for France in Act 1, Scene 3, Polonius tells him to be careful about what he says. He also tells Laertes to keep his trusted friends close. Polonius continues with different bits of advice which include avoiding quarrels, reserving judgment, and dressing with sophistication (in France). Polonius closes his comments to Laertes with the more famous lines: "Neither a borrower nor a lender be, / For loan oft loses both itself and friend, / And borrowing dulls the edge of husbandry." (I.iii.75-77)
Polonius' last bit of advice is perhaps more central to the play:
This above all--to thine own self be true,
And it must follow, as the night the day,
Thou canst not then be false to any man. (I.iii.78-80)
Laertes heeds this advice. When he returns to Denmark, he plans to avenge his father. Laertes is tricked by Claudius into conspiring against Hamlet but Laertes never hides his true feelings, one of the characters in the play that really doesn't act falsely. Claudius and Hamlet, on the other hand, are constantly conspiring and hiding their true feelings.