Following are some suggestions for removing marks and stains from wallpapered walls.
If you have a good washable wallpaper, you can simply wash it with a mild dishwashing liquid in warm water. For cleaning large areas, use a sponge mop with a new head on it.
To remove grease, lay several folds of blotting paper on the spot and hold a hot iron near it until the grease is absorbed.
Another grease removal method is to dust to the spots with talcolm powder, let it sit for a couple of hours and then brush off. Cornstarch or borax all can be used this way.
Marks that can't be washed away will often come off with a gum eraser (available at most art supply stores). Gum erasers crumble as you use them and are less likely to smear a mark than regular pencil erasers.
If you don't have a gum eraser handy, try rubbing marks and grease stains with a piece of bread. The bread acts as a gentle abrasive. It's crumbly, so keep a dustpan or vaccuum cleaner handy.
Remove stains with a mild solvent such as dry-cleaning fluid or rubbing alcohol. Test in an inconspicuous area to be sure the solvent doesn't damage the wallpaper.
Remove tape and other adhesives by passing a warm iron over them to melt the glue, then gently peel off. Use a mild dishwashing liquid in warm water to remove any glue that remains on the wall.
Source: The Old Farmer's Almanac 1999 Home Owner's Companion