There's a few methods you could use. The most common is meditation, go into a deep state of meditation, and you should start to go further back in your memories into past lives. Other methods are forms of sorting, like using a bowl of lunar water, or a black mirror to gaze at the face or events of a past life. I also believe you can in your dreams see past lives but I don't know how that works exactly.
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 :)
dang phone, i meant skrying not sorting lol, sorry about that.
@ shshgr, thanks, i remembered once i had a very vivid dream i was on the Titanic as it sunk [this was on the night of the 100 anniversary] and i shot awake once i hit the water [the time on the clock was 2:20 when i woke] so i thought it was possibly a past life, but i wasn't sure if that was a reliable method of regression.