What advice does Polonius give to Laertes in Hamlet and how does this advice still hold true today?

POLONIUS

Yet here, Laertes? Aboard, aboard, for shame!

The wind sits in the shoulder of your sail

And you are stayed for. There, my blessing with thee.

And these few precepts in thy memory

Look thou character. Give thy thoughts no tongue,

Nor any unproportioned thought his act.

Be thou familiar but by no means vulgar.

Those friends thou hast, and their adoption tried,

Grapple them unto thy soul with hoops of steel,

But do not dull thy palm with entertainment

Of each new-hatched, unfledged comrade. Beware

Of entrance to a quarrel, but being in,

Bear ’t that th' opposèd may beware of thee.

Give every man thy ear but few thy voice.

Take each man’s censure but reserve thy judgment.

Costly thy habit as thy purse can buy,

But not expressed in fancy—rich, not gaudy,

For the apparel oft proclaims the man,

And they in France of the best rank and station

Are of a most select and generous chief in that.

Neither a borrower nor a lender be,

For loan oft loses both itself and friend,

And borrowing dulls the edge of husbandry.

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.

Farewell. My blessing season this in thee. (I.iii.55-82)

Let us examine the above speech, full of excellent fatherly advice. He begins with "Give thy thoughts no tongue," which is advice not to always say what you're thinking, and then goes on to urge him not to always act on his thoughts. He then advises his son to keep the friends he has, and now make a bunch of new acquaintances willy-nilly, because loyalty can be hard to come by. He then urges him not to get into a fight -- "Beware of entrance to a quarrel" -- but then tells him to stand up for himself once he is in a fight. He urges him to speak little, but listen often; hear every man's opinion and reserve judgement. Then he gets a little more practical: Don't spend too much money on clothes, but what you do spend money on, make sure it is of excellent quality, because clothes are definitely important in France. Don't lend or borrow money from a friend, because you may often lose the money and the friendship.

But this above all: Be true to yourself. Be who you are. And if you are genuinely who you are, you will not be false to anybody else.

All of this is excellent advice and all of it holds true today, but particularly the last few lines: To thine own self be true. This is advice we constantly give our children and loved ones, because we know that it is the best path to happiness and self actualization.