Hot Water and white Vinegar is the easiest, safest, and most organice solution. It works well I think because of the acidic nature of the vinegar. After pulling off the top layer of paper (or vinyl) you are left with a tissue paper stuck to the drywall. I put the vinegar/water solution in a spray bottle and spray on the wall or apply with a sponge. Let it soak for a few minutes, and then scrape off with a putty knife. Then repeat with a sponge to remove traces of paste. The only downside is the smell if you don't like vinegar -- some find it a nice smell.