Q:

How can one know about their previous births?

How do I know what I was in my previous births?
There are many stories in Hindu mythology that mention the previous births of people. But my question is, even in those earlier times and in this current age, how can one know the previous birth of oneself or others? Or is knowing one's own or someone else's previous birth just a false concept? Is it just an illusion?
Also usually everybody forgets their past lives, so there must be some vidhya behind knowing one's past lives.
Can someone throw some light on how to attain this vidhya? Are there any Hindu texts on this topic?

A:

Patanjali Yoga Sutra III.18 says:

By making samyama on previous thought-waves, one obtains knowledge of
  one's past lives.

The basic idea is that every thought remains in a subtle form even after it is gone from our conscious mind. Thus one can though Yoga bring these
subtle thoughts back into the conscious mind. This can even be done for previous births.

A:

It is said that great meditation or tapasya brings new qualities in people. The person not only is able to control his senses but can also achieve more than that. 
There are examples in past about remembering past life like one story specified in Vishnu Purana of King Bharata, who after renouncing the worldly affairs took Sanyasa but later again got involved in loving a deer as his son. Since he thought all the time about the well being of deer and even on his death bed, he was born in his next birth as a deer. But then due to his meditation/tapasya of the previous birth, he was able to remember his past life. Here is an extract from Wikipedia article about this:

That sage was born as a deer in his next birth. The Vedic scriptures say that a man will be born in his next birth as that thing about which he was thinking most at the time of his death. Therefore wise men advise people to think of Supreme Personality of Godhead (Krishna, or any other Vishnu avatara) so that it will become a habit and thus after death achieve God's abode. As a result of his tapas and merits and having almost reached perfection the deer that was Bharata, could remember its past by the Lord's grace.

There is also a book called 'Bhrigu Samhita' written by sage Bhrigu. The Bhrigu Saṃhitā is an astrological (Jyotish) classic attributed to Maharishi Bhrigu during the Vedic period, Treta yuga. It is said that it was compiled by the Sage out of compassion for humanity so that humanity could cope with the pressures of its existence and move towards a more spiritual nature. The Bhrigu Samhita is said to contain predictions on one's current and future lives as well as information on the past life. These predictions will be accurate based on the actions (karma) of the questioner. Some part of Bhrigu Samhita is damaged and lost, so incomplete Bhrigu Samhita is still there in Hosiyarpur, India, where people still visit and try to find about their current, past and future births. I have already discussed about this book in my question here.
Finally, there are many examples of people remembering about their previous births even in present time. I remember a case which I read somewhere that states that a small boy who knew about how to speak in another language that was other than his native one and was able to remember and trace his previous life's parents. There are many other examples as well you can find on google. There is also a therapy under which people are even tried to remember their previous birth using Hypnosis by expert Hypnotists so as to find reason/solutions of their current problems.

A:

"Aparigraha sthairye janmakathamta sambodhaha'' (II Sutra 39, Patanjali Yoga Sutras)
Aparigraha = non-accumulation; Sthairye = established; Janmakathamta = of how births happen; Sambhodhaha = knowledge.
"Being established in non-accumulation gives knowledge of how births happen."
When you do not accumulate, then the knowledge of the previous births, knowledge of different species, the communication in you improves. Do you see what I am saying? When, a person says "more for me, more for me", then he is obsessed with fear, one does not know the eternal value of life. Life has been there for ages and will continue for ages to come.
Non-accumulating simply means confidence in one's existence. Confidence is in one's ability. Knowledge of one's self. You know how to make bread and know you can earn it. So you will not make bread for a week and store it for a whole year in your room. It becomes stale. In China there is a proverb "What you give, you gain more. Whatever you scatter you have it all. What you hold on to, you lose that also". When you scatter it all, it all comes to you. Everything is yours all over.
Who is very stingy? The one who is so afraid of himself, who has no idea of his strength. A person who is very selfish and stingy, he stores and stores accumulate. A very wealthy gentleman was on his death bed, but was on the telephone looking into the share market "how much I have?" He was going to die any moment, but was concerned about "what the share prices are? How much am I gaining, losing?" What is he going to do with the money now? He does not even believe that he has to leave everything here and go. If money transfer was possible, people would transfers all their money up there, before going. Lawyers would have had fun writing wills. They would transfer all their property up, take it with them. Fortunately or unfortunately this is not possible. People accumulate and accumulate and then they die. That does not mean that you should not save.
Not accepting or accumulating anything from anyone. What do we do? When people give us a compliment, we accept the compliment and then we begin to have a different idea about ourselves. They tell you, "You are so beautiful, so beautiful", and it goes to your head and you think you are beautiful. If ten people repeat the same thing it gets into your head and you lose all reality about who you are and what you are. Similarly, when you are on a trip to get things from others, that is called "parigraha." meaning "taking things". "Aparigraha" is the opposite of it, not trying to get anything from anybody.
Patanjali says, but what is the effect of it? Janma kathamta sambhodaha. You know, when you give things to people, it will give back something to you, some good vibrations to you. If you are very unhappy, then that day you give away something. Give some gifts to somebody. You will see how your consciousness will change, shift. Sometimes you accept things from someone, gifts or certain things and you feel miserable in life. In ancient days this rule stood very well. When someone accepts a gift, then the giver should be thankful. The giver is always thankful "Oh! I am thankful that you accepted the gift". So that was called as dakshina.

Taken from:
http://www.artofliving.org/yoga/patanjali-yogasutra/knowledge-sheet-71

