there's a ritual i read about in Scott Cunninghams Earth Magic where you use a large black mirror and some white candles [the candle's cannot be seen in the mirror] stand skyclad before the mirror and gaze into your eyes, you should start to see your past lives. [there might be a chant, but i can't remember]
meditation is another method. meditate and as you go into a deeper state, you should tap into past lives. i know of people who will say a chant, or tell themselves they wish to see past lives, and it happens. though it's not instant, and it will take practice to do.