How do you remember a past life?

This is a very intriguing and interesting question, and I am assuming you are referring to reincarnation. I have Buddhist friends who believe in reincarnation. According to the law of Karma, one may live many lives in this plane ie. this earthly world. The purpose of life in this earthly plane is to learn "lessons" on how to live a good and productive life and purify our souls so that one day we will be freed from "The Wheel of Reincarnation". If we make mistakes then we must come back into this earthly plane to correct those mistakes. However, we come back with all memories of past lives erased. For those who believe in reincarnation, it would be too easy if we knew the mistakes we had made, so we are given a "clean slate" and must start all over again.