One should render transcendental loving service to the Supreme Lord Kṛṣṇa favorably and without desire for material profit or gain through fruitive activities or philosophical speculation. That is called pure devotional service." One simply has to concentrate upon the lotus feet of Vāsudeva, Kṛṣṇa (sa vai manaḥ kṛṣṇa-padāravindayoḥ (S.B.9:4:18). Then the mind and senses will be controlled, and one can engage himself fully in the devotional service of the Lord.
(Srimad Bhagavatam-----8:17:2-3-----purport).

Tapaḥ is explained in the smṛti-śāstra as follows: manasaś cendriyāṇāṁ ca aikāgryaṁ paramaṁ tapaḥ. "Complete control of the mind and senses and their complete concentration on one kind of activity is called tapaḥ." Our Kṛṣṇa consciousness movement is teaching people how to concentrate the mind on devotional service. This is first-class tapaḥ.
(Srimad Bhagavatam-----6:1:13-14-----purport).

One has to concentrate one's understanding on the Supreme Truth, Kṛṣṇa or Vāsudeva. The word Vāsudeva indicates the Supreme Personality of Godhead, who is the cause of all causes.
(Srimad Bhagavatam-----5:12:8-----purport).

This remembrance of the Lord can be continued if we hear about Him constantly. It is therefore recommended in this verse: mukundācaritāgrya-sīdhunā. Sīdhu means "nectar." To hear about Kṛṣṇa from Śrīmad-Bhāgavatam or Bhagavad-gītā or similar authentic literature is to live in Kṛṣṇa consciousness. Such concentration in Kṛṣṇa consciousness can be achieved by persons who are strictly following the rules and regulative principles.
(Srimad Bhagavatam-----4:22:24-----purport).

By associating constantly with the Supreme Personality in his heart, Dhruva Mahārāja naturally became equal to the greatest, Brahman, by His association, and thus became the heaviest, and the entire universe trembled. In conclusion, a person who always concentrates on the transcendental form of Kṛṣṇa within his heart can very easily strike the whole world with wonder at his activities.
(Srimad Bhagavatam-----4:8:78-----purport).

One should not artificially try to see the form of the Lord while chanting Hare Kṛṣṇa, but when the chanting is performed offenselessly the Lord will automatically reveal Himself to the view of the chanter. The chanter, therefore, has to concentrate on hearing the vibration, and without extra endeavor on his part, the Lord will automatically appear.
(Srimad Bhagavatam-----4:8:53-----purport).

The word niyatena is very significant in this connection, for it indicates that one should execute the meditation practice as stated above. One should not manufacture a way of meditation on the Supreme Personality of Godhead, but should follow the authorized śāstras and personalities. By this prescribed method one can practice concentration upon the Lord until one is so fixed that he remains in trance, thinking always of the form of the Lord. The word used here is eka-bhūtena, which means "with great attention and concentration." If one concentrates on the descriptions of the bodily features of the Lord, one will never fall down.
(Srimad Bhagavatam-----4:8:51-----purport).

This process of concentrating on the different bodily parts of the Lord will gradually diminish the demoniac challenge of godlessness and bring about gradual development of devotional service to the Lord.
(Srimad Bhagavatam-----2:1:38-----purport).

Yoga means concentration of the mind detached from all other subject matter. And actually such concentration is samādhi, or cent percent engagement in the service of the Lord. And one who concentrates his attention in that manner is called a yogī.
(Srimad Bhagavatam-----1:9:23-----purport).

The yogī in Kṛṣṇa consciousness—even though he may be engaged in various activities while in material existence—remains always situated in Kṛṣṇa. This is confirmed in the Bhakti-rasāmṛta-sindhu (1.2.187) of Śrīla Rūpa Gosvāmī: nikhilāsv apy avasthāsu jīvan-muktaḥ sa ucyate. A devotee of the Lord, always acting in Kṛṣṇa consciousness, is automatically liberated. In the Nārada-pañcarātra this is confirmed in this way:
dik-kālādy-anavacchinne
kṛṣṇe ceto vidhāya ca
tan-mayo bhavati kṣipraṁ
jīvo brahmaṇi yojayet
"By concentrating one's attention on the transcendental form of Kṛṣṇa, who is all-pervading and beyond time and space, one becomes absorbed in thinking of Kṛṣṇa and then attains the happy state of transcendental association with Him."
(Bhagavad-Gita-----6:31-----purport).

At first, remembrance of Kṛṣṇa may be interrupted at intervals, but later remembrance proceeds uninterrupted. When remembrance is uninterrupted, it becomes concentrated and is called meditation. When meditation expands and becomes constant, it is called anusmṛti. By uninterrupted and unceasing anusmṛti one enters the stage of samādhi, or spiritual trance. After smaraṇa-daśā or samādhi has fully developed, the soul comes to understand his original constitutional position. At that time he can perfectly and clearly understand his eternal relationship with Kṛṣṇa. That is called sampatti-daśā, the perfection of life.
(Nectar of Instruction).

For one who always remembers Me without deviation, I am easy to obtain, O son of Pṛthā, because of his constant engagement in devotional service.
Undeviating concentration on the Supreme Lord is the first sign of pure devotion. In other words, a pure devotee is one who wards off all desires and thoughts not related to unflinching devotional service to the Supreme Lord.
(Renunciation through Wisdom).

Meditating on the lotus feet of the Lord acts like a thunderbolt on the mountain of dirt in the mind of the yogī. If a yogī wants to shatter the mountain of dirt in his mind, he should concentrate on the lotus feet of the Lord and not imagine something void or impersonal. Because the dirt has accumulated like a solid mountain, one must meditate on the lotus feet of the Lord for quite a long time. For one who is accustomed to thinking of the lotus feet of the Lord constantly, however, it is a different matter. The devotees are so fixed on the lotus feet of the Lord that they do not think of anything else.
(Srimad Bhagavatam-----3:28:22-----purport).

Constant meditation concentrated on the form of the Lord is called samādhi, ecstasy or trance. Samādhi means particularly concentrated attention, so one who has achieved the qualification of always meditating on the Personality of Godhead is to be understood to be always in trance and enjoying brahma-nirvāṇa, or brahmānanda.
(Srimad Bhagavatam-----4:6:39-----purport).

By chanting the Hare Kṛṣṇa mantra one immediately concentrates on the sound vibration and thinks of the lotus feet of the Lord, and very quickly one is elevated to the position of samādhi, or trance. If one goes on chanting the holy names of the Lord, which are not different from the Supreme Personality of Godhead, naturally his mind becomes absorbed in thought of the Lord.
(Srimad Bhagavatam-----4:8:44-----purport).