To concentrate the mind, one must observe a life of brahmacharya (celibacy) and not fall down. Celibecy is so important that even though one does not undergo any austerities, penances or ritualistic ceremonies prescribed in the Vedas, if one simply keeps himself a pure brahmacārī, not discharging his semen, the result is that after death he goes to the Satyaloka. Generally, sex life is the cause of all miseries in the material world. In the Vedic civilization sex life is restricted in various ways. Out of the whole population of the social structure, only the gṛhasthas are allowed restricted sex life. All others refrain from sex. The people of this age especially do not know the value of not discharging semen. As such, they are variously entangled with material qualities and suffer an existence of struggle only. 

praśāntātmā vigata-bhīr| brahmacāri-vrate sthitaḥ ||manaḥ saṁyamya mac-citto / yukta āsīta mat-paraḥ || (Bhagavad Gita 6.14)

with an unagitated, subdued mind, devoid of fear,  completely free from sex life, one should meditate upon Me within the heart and make Me the ultimate goal of life.

To realize this viṣṇu-mūrti within the heart, one has to observe complete abstinence from sex life; One has to practice controlling the mind and avoiding all kinds of sense gratification, of which sex life is the chief. In the rules of celibacy written by the great sage Yājñavalkya it is said:

karmaṇā manasā vācā /  sarvāvasthāsu sarvadā || sarvatra maithuna-tyāgo /brahmacaryaṁ pracakṣate||

“The vow of brahmacarya is meant to help one completely abstain from sex indulgence in work, words and mind – at all times, under all circumstances and in all places.” 

Therefore, Brahmacarya is taught from childhood when one has no knowledge of sex life. Children at the age of five are sent to the guru-kula, or the place of the spiritual master, and the master trains the young boys in the strict discipline of becoming brahmacārīs. Without such practice, no one can make advancement in any yoga, whether it be dhyāna, jñāna or bhakti. One who, however, follows the rules and regulations of married life, having a sexual relationship only with his wife (and that also under regulation), is also called a brahmacārī.  A householder brahmacārī is allowed controlled sex life because the cult of bhakti-yoga is so powerful that one automatically loses sexual attraction, being engaged in the superior service of the Lord. In the Bhagavad-gītā(2.59) it is said:

viṣayā vinivartante | nirāhārasya dehinaḥ || rasa-varjaṁ raso ’py asya | paraṁ dṛṣṭvā nivartate ||

Whereas others are forced to restrain themselves from sense gratification, a devotee of the Lord automatically refrains because of superior taste. Other than the devotee, no one has any information of that superior taste.