"Anyone who quits his body, at the end of life, remembering Me, attains immediately to My nature; and there is no doubt of this." (Bg. 8.5) One who thinks of Krsna at the time of his death goes to Krsna. One must remember the form of Krsna; if he quits his body thinking of this form, he approaches the spiritual kingdom. Mad-bhavam refers to the supreme nature of the Supreme Being. The Supreme Being is sac-cid-ananda-vigraha--eternal, full of knowledge and bliss. Our present body is not sac-cid-ananda. It is asat, not sat. It is not eternal; it is perishable. It is not cit, full of knowledge, but it is full of ignorance. We have no knowledge of the spiritual kingdom, nor do we even have perfect knowledge of this material world where there are so many things unknown to us. The body is also nirananda; instead of being full of bliss it is full of misery. All of the miseries we experience in the material world arise from the body, but one who leaves this body thinking of the Supreme Personality of Godhead at once attains a sac-cid-ananda body, as is promised in the fifth verse of the Eight Chapter where Lord Krsna says, "He attains My nature."

The process of quitting this body and getting another body in the material world is also organized. A man dies after it has been decided what form of body he will have in the next life. Higher authorities, not the living entity himself, make this decision. According to our activities in this life, we either rise or sink. This life is a preparation for the next life. If we can prepare, therefore, in this life to get promotion to the kingdom of God, then surely, after quitting this material body, we will attain a spiritual body just like the Lord's. ...."In whatever condition one quits his present body, in his next life he will attain to that state of being without fail."

Bhagavad-gita, Introduction
 
 

"The unalloyed devotees of the Supreme Lord who are totally surrendered souls do not care when they leave their bodies or by what method. They leave everything in Krsna's hands and so easily and happily return to Godhead. But those who are not unalloyed devotees and who depend instead on such methods of spiritual realization as karma-yoga, jnana-yoga, hatha-yoga, etc., must leave the body at a suitable time and thereby be assured whether or not they will return to the world of birth and death.

If the yogi is perfect, he can select the time and place for leaving this material world, but if he is not so perfect, then he has to leave at nature's will. The most suitable time to leave the body and not return is being explained by the Lord in these verses...

Those who know the Supreme Brahman pass away from the world during the influence of the fiery god, in the light, at an auspicious moment, during the fortnight of the moon and the six months when the sun travels in the north.

When fire, light, day and moon are as mentioned, it is to be understood that over all of them there are various presiding deities who make arrangements for the passage of the soul. At the time of death, the jiva sets forth on the path to a new life. If one leaves the body at the time designated above, either accidentally or by arrangement, it is possible for him to attain the impersonal brahmajyoti. Mystics who are advanced in yoga practice can arrange the time and place to leave the body. Others have no control--if by accident they leave at an auspicious moment, then they will not return to the cycle of birth and death, but if not, then there is every possibility that they will have to return. However, for the pure devotee in Krsna consciousness, there is no fear of returning, whether he leaves the body at an auspicious or inauspicious moment, by accident or arrangement.

The mystic who passes away from this world during the smoke, the night, the moonless fortnight, or the six months when the sun passes to the south, or who reaches the moon planet, again comes back.

In the Third Canto of Srimad-Bhagavatam we are informed that those who are expert in fruitive activities and sacrificial methods on earth attain to the moon at death. These elevated souls live on the moon for about 10,000 years (by demigod calculations) and enjoy life by drinking soma-rasa. They eventually return to earth. This means that on the moon there are higher classes of living beings, though they may not be perceived by the gross senses.

According to the Vedas, there are two ways of passing from this world--one in the light and one in darkness. When one passes in light, he does not come back; but when one passes in darkness, he returns.

The same description of departure and return is quoted by Acarya Baladeva Vidyabhusana from the Chandogya Upanisad (5.10.3-5). In such a way, those who are fruitive laborers and philosophical speculators from time immemorial are constantly going and coming. Actually they do not attain ultimate salvation, for they do not surrender to Krsna.
The devotees who know these two paths, O Arjuna, are never bewildered. Therefore be always fixed in devotion.

Krsna is here advising Arjuna that he should not be disturbed by the different paths the soul can take when leaving the material world. A devotee of the Supreme Lord should not worry whether he will depart either by arrangement or by accident. The devotee should be firmly established in Krsna consciousness and chant Hare Krsna. He should know that concern over either of these two paths is troublesome. The best way to be absorbed in Krsna consciousness is to be always dovetailed in His service, and that will make one's path to the spiritual kingdom safe, certain, and direct. The word yoga-yukta is especially significant in this verse. One who is firm in yoga is constantly engaged in Krsna consciousness in all his activities. Sri Rupa Gosvami advises that one should be unattached in the material world and that all affairs should be steeped in Krsna consciousness. In this way one attains perfection. Therefore the devotee is not disturbed by these descriptions because he knows that his passage to the supreme abode is guaranteed by devotional service."

Bhagavad-gita 8:23-27

Bhaktivedanta Book Trust. HDG A.C. Bhaktivedanta Swami Srila Prabhupada.