What is reincarnation?
Reincarnation is the return of the spiritual principle into a new physical envelope. For a human being, this envelope is always a human body. Theoretically, someone can reincarnate on another planet. Each life lived is as long as it needs to be -- whether three years or ninety -- and is designed to expose the spiritual principle to the physical elements in order to accelerate its evolution toward the One. If we did not incarnate, it would be impossible for our soul to understand itself and for us to understand our soul and life. We can only understand that which we can see and experience.
In general, young children of two or three years old often get vague recollections of their past lives, but the memories fade around seven or eight years old. Some of our skills can disappear from one life to another, or be hidden, when the spirit wants to learn other skills, but many never really disappear and can blossom in a further life -- especially later in the cycle.
Since the beginning of all religions, reincarnation has been considered as an alternative to death. The first who have talked about it were the Egyptians, and many passages of The Book of the Dead mention the afterlife and being reborn. Better known are the tenets of Buddhism and Hinduism. In Tibet, when it comes to reincarnation, we undeniably think of the Dalaï Lama who is thought to be the first Dalaï Lama in his latest incarnation.
Reincarnation is interesting in that it places responsibility directly onto the individual as well as helping us to understand and to accept the worst ordeals. Not as a condition of sin but as tests for those who are not perfect yet, to make them think about past mistakes and to give them the chance to evolve their soul?
Examining your past lives is a way of becoming more aware of your evolution. You can learn from past experiences, so that if a similar situation comes up in your current lifetime you have more information with which to handle the event. I caution you, however, that past life regressions are a particularly easy place for the Ego to get stroked by frauds, and Illusion. I always wonder why when someone talks about their recent regression, they always seem to be someone famous. I mean, wasn't anyone just a serf of a bond-servant to a Roman Administrator or something dull like that?
Past life regressions can also be traumatic. Say you are afraid of water, and during a regression you discover that in past lives you frequently drowned -- and you end up experiencing one of those deaths. Ick. My brain hurts just thinking about it.
I believe that as your soul evolves, accessing your past lives becomes easier to do -- and that this information will come to you through dreams, meditation, and intuitions. For me, one of the best resources for understanding has been the Michael Teachings, which are available in several books (see the Booklist, under Channeling) and at this website. I also believe that as you go through your lives, the time you spend 'between' lives gets longer, as you have more data to collate, sift through, organize and then assimilate.
If you are going to explore your past lives, it may be a very good idea to do so with a partner. They can provide a safety net for you, a Guide into scary and perhaps dangerous waters (see my re-living a death comment above).