The idea of having more than one life in Buddhism is regarded not just as a theory but a truth that can be experienced and verified If this is so, what was the technique used by the Buddha to recollect past lives? What is the exact technique that leads to remembering one's past lives according to Buddhism?
According to my teachers (Korean Zen and Tibetan Vajrayana), because Buddhism explicitly denies existence of soul (see
anatta), the concept of rebirth does not refer to reincarnation of the same person, but rather to propagation of information from one life to another.
Recollection of past lives then is not to be understood literally but as a metaphor for clearly seeing patterns of karma acting out from one generation to another. This requires shedding an irrational belief in existence of "I" as a solid entity, subject of experience, and independent agent of action.
The technique for achieving this level of insight is known as
vipassana or "special insight meditation", to be preceded by "calm abiding meditation",
samatha, in order to attain the necessary clarity of mind.
Technically, the memory of past lives requires the four meditative stages to be accomplished. See my answer to this thread: What exactly is Jhana?
"Visuddhimagga" Chapter XII can be used as the reference. It begins with
Now, in order to perfect those kinds of direct-knowledge the task must be undertaken by a meditator who has reached the fourth jhána in the earth kasióa, and so on.
Here it describe 5 kinds of "direct- knowledge". The 4th one is the knowledge of recollection of past lives;
According to the text the technique is quite lengthy (hence not going to past here) and very well described.
Apart from that, I have read /listened to various "Dhamma" talks regarding this subject. According to those, it is required to have a deep state of "samadhi" to begins with.
Then you can train your mind to remember the past. You could start with remembering things backwards. For example, what was I doing before now, then before that, then before that...
When you got stuck, just come back to the meditation, develop the "samadhi" again and try training again.
Please have a look at these could of videos as well.
Hope this helps.
According to the Maha-Assapura Sutta translated by Thanissaro Bhikkhu, a practitioner works on The Three Knowledges, one of which is recollection of past lives, after obtaining all four Jhanas..
"And furthermore, with the abandoning of pleasure and stress — as with the earlier disappearance of elation and distress — he enters and remains in the fourth jhana: purity of equanimity and mindfulness, neither-pleasure nor stress.
"With his mind thus concentrated, purified, and bright, unblemished, free from defects, pliant, malleable, steady, and attained to imperturbability, he directs and inclines it to knowledge of the recollection of past lives.