Bg 2.60

yatato hy api kaunteya
puruṣasya vipaścitaḥ
indriyāṇi pramāthīni
haranti prasabhaṁ manaḥ
Word for word: 
yatataḥ — while endeavoring; hi — certainly; api — in spite of; kaunteya — O son of Kuntī; puruṣasya — of a man; vipaścitaḥ — full of discriminating knowledge; indriyāṇi — the senses; pramāthīni — agitating; haranti — throw; prasabham — by force; manaḥ — the mind.
Translation: 
The senses are so strong and impetuous, O Arjuna, that they forcibly carry away the mind even of a man of discrimination who is endeavoring to control them.
Purport: 

There are many learned sages, philosophers and transcendentalists who try to conquer the senses, but in spite of their endeavors, even the greatest of them sometimes fall victim to material sense enjoyment due to the agitated mind. Even Viśvāmitra, a great sage and perfect yogī, was misled by Menakā into sex enjoyment, although the yogī was endeavoring for sense control with severe types of penance and yoga practice. And, of course, there are so many similar instances in the history of the world. Therefore, it is very difficult to control the mind and senses without being fully Kṛṣṇa conscious. Without engaging the mind in Kṛṣṇa, one cannot cease such material engagements. A practical example is given by Śrī Yāmunācārya, a great saint and devotee, who says:

yad-avadhi mama cetaḥ kṛṣṇa-pādāravinde
nava-nava-rasa-dhāmany udyataṁ rantum āsīt
tad-avadhi bata nārī-saṅgame smaryamāne
bhavati mukha-vikāraḥ suṣṭhu niṣṭhīvanaṁ ca

“Since my mind has been engaged in the service of the lotus feet of Lord Kṛṣṇa, and I have been enjoying an ever new transcendental humor, whenever I think of sex life with a woman, my face at once turns from it, and I spit at the thought.”

Kṛṣṇa consciousness is such a transcendentally nice thing that automatically material enjoyment becomes distasteful. It is as if a hungry man had satisfied his hunger by a sufficient quantity of nutritious eatables. Mahārāja Ambarīṣa also conquered a great yogī, Durvāsā Muni, simply because his mind was engaged in Kṛṣṇa consciousness (sa vai manaḥ kṛṣṇa-padāravindayor vacāṁsi vaikuṇṭha-guṇānuvarṇane).