I'm not overly familiar with Hamlet, but I remember somebody mentioning that the phrase, "To Thine Own Self Be True" was intended as a bit of mockery, and the character isn't exactly noted for his philosophical depth..

http://www.shmoop.com/hamlet/polonius.html

>Polonius and Comedy

>Polonius is a pretty foolish character. There's no getting around it: he is self-absorbed, long-winded, and dull. He's also, when played by the right actor, completely hilarious. It's all about finding ways to make Polonius's long speeches as amusing as they can be. He likes to give the "when I was your age" speeches, as we see when he tells Hamlet that he was in love once, too, back in the day. He also dishes lame advice like it's going out of style. He's like an over-eager parent who gives unwanted opinions.

>For all these reasons, Polonius can be a source of comic relief to a weighty play. Check out the scene where Hamlet's directing the players (actors). Polonius is hilarious. When one of the players delivers a heart-wrenching rendering of Priam's death and the hullabaloo to follow, Polonius interrupts to say (and we are not making this up), "This is too long."

>Polonius's relationships with his children are of particular interest. His decision to spy on his son while Laertes is off at college is in keeping with the play's theme of "Lies and Deceit," as is his use Ophelia to spy on Hamlet. As comical and ridiculous as Polonius is, his elaborate attempts to keep tabs on Laertes and Ophelia remind us that father's cannot always be trusted to care for their children. This is especially true when Polonius pays Reynoldo to spread rumors about Laertes so that Laertes will confide in Reynoldo, who can then report back to Polonius

Whenever someone reminds me of the "to thine own self be true" line, I ask them if they realize that the fellow who speaks this line meets his demise because he is hiding behind a tapestry, spying on our hero.

Most people have wonderful advice for others, and very little ability to apply it to their own lives.