I've never seen a meditation or ritual to do so, I've connected with past lives through artistic mediums (though, I am no artist, it just all pops into my head). Most times, it's simply subconscious. Your past lives pop into your head and you start having dreams that play out like memories (excuse the Assassin's Creed reference), and it's almost like laying in an Animus. Basically, you see and play everything out through this collective conscious thought. Everything seems robotic at first, with people giving generic responses, until you actually start playing the story out.