Reincarnation transmigration of the soul

The most fundamental information about reincarnation is found in Bhagavad-gita. In Chapter 2, Krishna explains in a rational, simple and clearly understandable way how the soul travels from body to body. This fact is not a question of belief but is relatively easy to understand and to accept with logical conclusions.

The prerequisite for this understanding of reincarnation is that one understands the difference between the body and the soul.

"That which pervades the entire body you should know to be indestructible. No one is able to destroy that imperishable soul. " (Bhagavad Gita 2.17)

"For the soul there is neither birth nor death at any time. He has not come into being, does not come into being, and will not come into being. He is unborn, eternal, ever-existing and primeval. He is not slain when the body is slain." (Bhagavad Gita 2.20)

"The soul can never be cut to pieces by any weapon, nor burned by fire, nor moistened by water, nor withered by the wind.(Bhagavad Gita 2.23)

Thus, the soul and the body are two different things. The body is temporary and the soul is eternal.

Although the nature of the soul is beyond the scope of measurable material interactions, one can realize its presence with the help of the following example:

"O son of Bharata, as the sun alone illuminates all this universe, so does the living entity, one within the body, illuminate the entire body by consciousness." (Bhagavad Gita 13.34)

There are various theories regarding consciousness. Here in Bhagavad-gita the example of the sun and the sunshine is given. As the sun is situated in one place, but is illuminating the whole universe, so a small particle of spirit soul, although situated in the heart of this body, is illuminating the whole body by consciousness. Thus consciousness is the proof of the presence of the soul, as sunshine or light is the proof of the presence of the sun. When the soul is present in the body, there is consciousness all over the body, and as soon as the soul has passed from the body, there is no more consciousness. This can be easily understood by any intelligent man. Therefore consciousness is not a production of the combinations of matter. It is the symptom of the living entity. The consciousness of the living entity, although qualitatively one with the supreme consciousness, is not supreme because the consciousness of one particular body does not share that of another body. But the Supersoul, which is situated in all bodies as the friend of the individual soul, is conscious of all bodies. That is the difference between supreme consciousness and individual consciousness. (Purport, Srila Prabhupada)

Although reincarnation constantly takes place because the soul is changing its various bodies, in the West the term reincarnation is mainly referred to as the final change of the soul into a new body.
"As the embodied soul continuously passes, in this body, from boyhood to youth to old age, the soul similarly passes into another body at death."(Bg 2.13)

"As a person puts on new garments, giving up old ones, the soul similarly accepts new material bodies, giving up the old and useless ones."(Bg 2.22)
At the time of death the spirit soul leaves the gross body together with the subtle body (astral body-mind, intelligence and false ego)and is "born again" into the womb of a female body according to the law of karma (either immediately or after some time).

One's next birth is determined by one's consciousness at the time of death (yam yam vapi smaran bhavan...Bg 8.6) because everything we do or think in our present life leaves an impression in the mind and the summary of these impressions influences our consciousness at the time of death.

It is important to understand that one may have to accept any type of body after leaving one's human body. If one's human existence is without the cultivation of spiritual knowledge one has to enter lower forms of life.

So how does one purify his consciousness so that one can have a better life next time or avoid reincarnation all together?

Lord Krishna gives us the answer:

"And whoever, at the time of death, quits his body, remembering Me alone, at once attains My nature. Of this there is no doubt. "(Bhagavad Gita 8.5)

The only way we will remember Krishna at the time of death is to remember him during our life.
The best way to remember Him is to chant His holy names-the Hare Krishna Maha Mantra. It is the yajna (sacrifice) for this age.