1. Your protagonist's mentor must be old. Not kind of old, ancient. No one's sure of his exact age. This is because in order to be wise, you must be wizened. After all, who ever heard of an old fool or a a young genius?
2. He must have a long white/grey beard. Unless he is a mythological creature, like a dragon. In which case he must be able to show his age in other ways, like the protagonist being able to see his years in his eyes. Telling someone's age by looking into their eyes is a skill that most fictional characters possess.
3. Regardless of age and appearance, he must have a grand countenance. If your mentor is a woman, she must be old and hunched over and ugly. She does not have the presence and grandeur that a male mentor does.
4. Your mentor must know everything. He has the answer to anything and always knows what is going to happen. He will warn your protagonist of what not to do, only to later reveal that he knew that it was inevitable all along.
5. He must be...vague. Whenever he gives the protagonist advice or warning, he only gives the barest hint of what is to come or what should be done. He never actually imparts any solid, useful information.
6. If the story deems that your protagonist must not yet know certain information, but it is knowledge that your protagonist is desperate for, his mentor will conveniently avoid answering the question.
7. The mentor of your story should have a whimsical side. This usually shows itself through a sense of humour. Telling jokes, or changing the subject to a favourite sport or activity, is a great way to avoid answering though all-important questions that the protagonist keeps asking.
8. Your mentor should throw around phrases like, "I cannot answer that," or "It is your destiny," or "I warned you that this might happen."
9. Your mentor dies. This throws your protagonist into emotional peril, because how is he meant to go on without the aid of this gibbering genius?
10. Even though the mentor is dead, all the advice and truths and answers that your protagonist sought through the story are finally revealed. The protagonist prevails! And the old git of a guardian proves to be more useful in death than in life.
I have imparted what wisdom I can.