Answer: A lot! Don't let the student practice in a way that he or she gets worse! I've often felt that someone like Bobby Knight would be one of the best chess coaches in the world although his style of "coaching" is not good for young kids (he's a little too intense for the average chess player).
Here are a few of my favorite Bob Knight quotes. Also I apologize for using sports analogies (I'm tired of people doing this).
Bonus: Find the quotes that are similar/redundant.
"The will to succeed is important, but what's more important is the will to prepare."
"You don't play against opponents, you play against the game of basketball."
"The key is not the will to win... everybody has that. It is the will to prepare to win that is important."
"Most people have the will to win, few have the will to prepare to win."
"People change over the years, and that changes situations for good and for bad."
"Mental toughness is to physical as four is to one."
"I have always maintained that in basketball the importance of the mental to the physical is about four to one."
"If I came in to recruit your son, I would tell you, your wife, and your son, that I will be the most demanding coach your son can play for."
"I don't intend to retire."