The eyes have been called "the windows of the soul." There is a belief that every moment, from the instant we first came into the physical realm in our very first incarnation, can be found within a person's eyes. While there are endless methods to discover past lives, the method I'll give here is one of the easiest, but by no means the most effective. Usually this methods results in uncovering the lives the two people have shared. And you need a willing partner to do this one. But it is simple and can be done anywhere with little to no preperation.
Sit close to your partner in a comfortable position (I've always done this ritual sitting cross-legged on the ground, but body positioning isn't terribly important for this exercise). Look into each others eyes. Allow a moment or two for giggling and smiles as we're unaccustomed to closely gazing into another's eyes for any length of time. When you begin to settle down, try to relax yourself, breathing regularly and clearing your mind. As you do so, images will become to come into your head. Let them come, not thinking about them, but just watching. Timelines and characters will change. Occasionally, the entire present world will fade around you and you will find yourself in another place or time for a moment or two. You can discuss the images as they come to you, but that will actually ground you in this "time." When you're done, (you'll know; suddenly you'll look away, start laughing... the serious mood will be lost) give each other a hug and stomp your feet. The hug is because you have been making use of each other's energy to view the lifetimes and any energy use is always accompanied by a thank you. Stomping your feet will help ground you and keep you from drifting off through the timelines unintentionally.
While there are other, more effective methods to discovering past lives, this is one of the exercises that I give new students as it also teaches a person to follow thier instincts and begins to hone latent psychic abilities. I've known two people who could unlock past lives through the Tarot but, as I've only just begun my Tarot studies, I have no idea how they do it (nor would I necessarily understand if someone explained it!). I've also been regressed by a liscensed hypnotherapist and have learned the skills necessary to do self-regressions as well to help others do it. But as I am not a liscensed hypno-therapist, I don't feel comfortable sharing these techniques as I'm certain there are others more qualified to do so.