Bg 6.34

cancalam hi manah Krishna
pramathi balavad drdham
tasyaham nigraham manye
vayor iva su-duskaram

Word for word: 
cancalam — flickering; hi — certainly; manah — mind; Krishna — O Krishna; pramathi — agitating; bala-vat — strong; drdham — obstinate; tasya — its; aham — I; nigraham — subduing; manye — think; vayoh — of the wind; iva — like; su-duskaram — difficult.

Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
The mind is restless, turbulent, obstinate and very strong, O Krishna, and to subdue it, I think, is more difficult than controlling the wind.

Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
The mind is so strong and obstinate that it sometimes overcomes the intelligence, although the mind is supposed to be subservient to the intelligence. For a man in the practical world who has to fight so many opposing elements, it is certainly very difficult to control the mind. Artificially, one may establish a mental equilibrium toward both friend and enemy, but ultimately no worldly man can do so, for this is more difficult than controlling the raging wind. In the Vedic literature (Katha Upanisad 1.3.3–4) it is said:

atmanam rathinam viddhi
sariram ratham eva ca
buddhim tu sarathim viddhi
manah pragraham eva ca

indriyani hayan ahur
visayams tesu gocaran
atmendriya-mano-yuktam
bhoktety ahur manisinah

“The individual is the passenger in the car of the material body, and intelligence is the driver. Mind is the driving instrument, and the senses are the horses. The self is thus the enjoyer or sufferer in the association of the mind and senses. So it is understood by great thinkers.” Intelligence is supposed to direct the mind, but the mind is so strong and obstinate that it often overcomes even one’s own intelligence, as an acute infection may surpass the efficacy of medicine. Such a strong mind is supposed to be controlled by the practice of yoga, but such practice is never practical for a worldly person like Arjuna. And what can we say of modern man? The simile used here is appropriate: one cannot capture the blowing wind. And it is even more difficult to capture the turbulent mind. The easiest way to control the mind, as suggested by Lord Caitanya, is chanting “Hare Krishna,” the great mantra for deliverance, in all humility. The method prescribed is sa vai manah Krishna-padaravindayoh: one must engage one’s mind fully in Krishna. Only then will there remain no other engagements to agitate the mind.