Seeing past lives in your dreams also works, as Nekoshema mentioned. I find this method to be the easiest for me, but everyone is different.
All you really have to do for this technique is simply go to bed with the intention of remembering one of your past lives. It may help to say something like:
"When I go to sleep, I will dream about my past life, and I will remember it in the morning."
This method can however take quite a few tries and may be frustrating at first. Unless you can lucid dream, you will probably only get random bits and pieces that don't seem to connect at all.
It's best to keep a journal and write them down, as they will probably make more sense each time you get more information :)