I never tried this before, but I found it on the internet a long time ago when I was wondering about some things:
"Take a dark colored bowl ( it must be dark on the inside) and fill it with water. Get yourself comfort sitting on the floor or bed or the ground outside. Relax, calm down and gaze at the water. Think of the water moving and flowing, then think of the ebb and flow of time. Looking into, not at the water is important here. As you do this think back along this lifetime. Let your memories flow backward. Don't try to concentrate on them, just watch them flow as if they were flickering through the water in the bowl. Watch them move farther and farther back and do not, I repeat, do not allow yourself to place a limit on them as they flow. If you do you will likely hit the beginning of this life and move no further. Let them flow as they will and only just take in what you see, don't attempt to analyze until afterwards. As the vision flows back just keep thinking "Before", or "Earlier", or something along those lines. Let your mind go so that you can stop thinking of yourself as the body you are in and the life you have had here. It may take some time, it may not work at all, but at the very least you should get some interesting impressions from the visions in the bowl. It's a start anyway."