Casting Polonius in a demeaning light is a common danger. While he is a blowhard, and he does spout aphorisms that were, even in the 16th century, clichés, his clichés constitute sound advice and his observations prove themselves prophetic.
Polonius may be elderly and demented, but he must have been at least politically adept. He admits that he is not a man of great prestige, and yet he has risen to be counselor to the King. Presumably, he counseled King Hamlet as well.
An actor portraying Polonius should address the question of whether he is a devoted father or a ruthless politician. Does he sacrifice Ophelia to his ambitions and/or his fear of being discarded by the King? Does he send Reynaldo to spy on Laertes because he cares about his son, or is he worried about what Laertes' possible behavior might reflect back on his own character? Is he more concerned with his position in Denmark than with the welfare of his children? Is he then the victim of his own contrivances?