In Act 1, Scene 3 of Hamlet, Laertes is about to embark for France to attend the university. His father Polonius gives him the following famous lines of advice.

Give thy thoughts no tongue,
Nor any unproportion'd thought his act.
Be thou familiar, but by no means vulgar.
Those friends thou hast, and their adoption tried,
Grapple them to thy soul with hoops of steel;
But do not dull thy palm with entertainment
Of each new-hatch'd, unfledged comrade. Beware
Of entrance to a quarrel; but being in,
Bear't that the opposed 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 express'd 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.

Although Prince Hamlet regards Polonius with contempt, and although the old man does seem foolish on more than one occasion, his advice to his son is full of sound practical wisdom. No doubt this wisdom is coming from Shakespeare himself and he is only using Polonius as his spokesman. For instance, Shakespeare twice cautions young people to be very careful about what they say to others.

Give thy thoughts no tongue

And:

Give every man thy ear, but few thy voice

What Shakespeare's character Polonius says about borrowing or lending money is also good advice, as many people can attest from experience.

Neither a borrower nor a lender be;
For loan oft loses both itself and friend,
And borrowing dulls the edge of husbandry.

What Polonius means by borrowing dulling the edge of husbandry is that if you get into the habit of living on borrowed money you can dig yourself into a hole that will be hard to climb out of. Borrowing also tends to make a person careless about earning, saving, spending, and budgeting. 

The best advice of all comes last and is the hardest to follow:

This above all, to thine own self be true

It is hard to follow because it is hard to understand your own self. But it is worth the effort to do so. If you understand yourself you will make better career decisions, a better marital choice, better friends, and end up leading a more satisfying and prosperous life. Robert Frost wrote about making a choice between two diverging roads in his famous poem "The Road Not Taken." He made his decision based on his knowledge of himself. There are many times in life when we have to decide something for ourselves and no one can advise us. We come to a lot of diverging roads, or what William James called "crossroads situations," in life. Polonius says, "Take each man's censure, but reserve thy judgment." By "censure" he means advice. A lot of people will offer you free advice, and you should listen to them--but in the end you should make your own decisions.

Buddha is quoted as having told a disciple:

Do not believe anything anybody tells you, including anything I tell you, unless it agrees with your own experience and your own common sense.