how we focus on what not to focus?

A guru told his disciple to meditate on anything, "But do not let the monkey come into your thoughts. Meditate on anything you wish, just not the monkey." 

"That's easy," thought the disciple. He sat day and night but all he could think of was the monkey. He closed his eyes and the monkey appeared. Traumatised by the monkey, he came to his guru in the morning and told him that while he tried not to think of the monkey, all he had done the entire time was to think of the monkey! The guru was not surprised.

He knew the power of exclusion. Then why did he suggest the monkey to the disciple? "In reality one cannot live with just what one wants to live with. To be able to steer your mind away from that which is not desired to that which is desired, is the true art of meditation," the guru pointed out.