To begin, I must first explain the way I feel about the universe (at least vaguely). We as 'human' souls were created by God (a collection of energies, male and female), and lived happily as souls for a period of time (even though it doesn't really exist there

There is no possible way a soul can learn all that it needs in one life, and as such, must take on many lives as many things (though I personally believe that all humans stay humans, and that animal reincarnation is left to animals -- again, that's just my personal belief). Some must learn the strife of poverty and the loneliness of wealth, the the sadness of disability and the triumph of success, all of which must be learned in the span of the Soul. What each Soul's journey is, is known to that of the Soul and that of the Guide(s), who assist you in your goals to learn (while themselves, learning second-hand). At this point of completion, I could go on to explain my views on the God and Goddesses of the Past, but those are my own beliefs and in such a heavily Wiccan community, I don't want to spark any debates.
In a book I read it described this completion of personal goals as a bucket and a thimble. If you fill each with water, which is fuller? The bucket or the thimble? NEITHER! They're both completely full, one simply 'chose' to hold more.
One can go about meeting past lives in a plethora of ways, and I'll attempt to share the ones I am familiar with here. As with any work in the Craft, to some, things easily come, whereas others may work their entire life to achieve the same results as a 'Natural.' I, with my NUMEROUS lives as a witch, have the luxury of being a Natural when it comes to calling them, remembering them. That being said, you must bear with me when I try to explain, as most of these things just COME to me.
(Warning: My own beliefs) You must understand that, while yes, you are a Soul incarnate, the Soul is a TREMENDOUS thing, and can no way entirely fit into the body of a measly human! This is the reason that I find past lives can appear as any other spirit can (if you simply call to them). You'll also find that I HATE making things more difficult than they should be, and you CERTAINLY won't find a guided meditation, if that's what you're looking for. I don't deal with that, as it takes way too much time and energy for me. It is this reason that I simply called out to my lives, and just as a God or Goddess may, they take a leap through the veil and come to you (unless they're busy). Understand that the first time, knowing nothing of your past lives (and if you have inklings, don't hesitate to go on them), you may find it difficult to pull one through. The more accustomed you get to the process, as well as connect to the specific energy of your lives, the easier it is to call them back. If you have the ability to see spirits, I would suggest this way first, though they do seem to have a different make-up than your average trapped Spirit. I actually see past lives better than average Spirits.
If your ability to see spirits is either weak or dormant entirely, I suggest using a pendulum or a (don't be alarmed) Ouija Board. When used correctly, a Ouija Board paired with a past life can be a WEALTH of information. A pendulum slightly less, and should be used after having already gotten acquainted with a past life.
Dreams are another phenomenal way to learn of your previous lives, and it works in generally the same manner as above. Call them to you, or simply ask your Guide to show you things of the past, memories, perhaps even bring yourself and a life of yours together somewhere to chat. When I have these dreams they often begin in a very peculiar manner and you can always tell it's something entirely different from a regular dream. Often I find that a dream will begin with familiar faces and scenery and, as the dream progresses, scenes will change and faces into those they wore before. It's easier to comprehend who and what is going on that way. That may be a personal thing, though. It seems like something my Guide would do, as she sort of babies me, occasionally.

The last way I learn of past lives is by sheer memory. It's unique the way memories of lives past come into your mind. Usually I find it happens when I listen to music and have a time to be very calm, like riding in the car, looking off into space. Given the opportunity, the seal can be broken, and then they will just flood in at all times! The hardest part about this is BREAKING the SEAL!! People can work for years to break the seal that blocks between this life and that. And also don't be surprised if certain things are beyond your memory. I have found that in certain instances, that there are things I am not supposed to learn about a life until later in this one. Trust your soul and trust your Guide, as they won't show you anything that you shouldn't see.
These are the ways that I have learned to contact my past lives, and in this way, I have learned about almost 20 of my lives. Not only their names and where they lived, but their jobs (usually they were just witches), hobbies, loves, friends, goals and failures. They are truly a wealth of knowledge on their own, be it perhaps book smarts (depending on the lives you've had), but also in the sense category.... They have lived their life, and they have learned from other lives, just as you are now. Have faith in them, but also realize that what they suggest, what they have done, should not always be done without question or idolized, as they've not had all of the overall experience your Soul may have gained since the life they lived.
Be warned and good luck! I hope I've been of some assistance.
Lovingly,
Qedava the Grey
P.S.
If you know a good and trusted medium, you can also ask them to Channel a life since past. I've done this before, but be prepared, as the mannerisms you share may at first be startling. Especially in the way they speak, the way they move, and their humor!