For grease spots apply a paste of cornstarch and water to the stain and allow to dry. Brush or vacuum off the cornstarch after a day, the grease should be gone.
If you have a little nic or tear, you can conceal it by coloring in the damaged area. This won’t work for big chunks, but the odd small nic will be disguised nicely. Simply use a felt marker that best matches and color the offending area in.
Try wadding up a bit of bread and using that as an “eraser”, rub out the stains.
For leftover rolls, try using them to line drawers as well as closet and cupboard shelves. Great way to use up supplies without tossing them out. Also make sure to keep some extra on hand “just in case” you have to make a small repair job on the walls.
For crayon marks or greasy stains, try removing as much excess wax as you can (use a blunt knife or spoon) then cover the spot with two sheets of paper towel and iron over it with a warm iron. Also see this page for more tips.
For washable varieties, you can try this homemade cleaner: 1 cup ammonia, 1 tsp dish detergent, 1 gallon water. Also a pail of water with a bit of dish detergent will work too. Test a small area first to make sure paper won’t be damaged with whatever cleaning solution you choose.
If It’s lifting at the seams, first brush under the seams to remove any dirt. With a small paintbrush, apply wallpaper adhesive behind the seam on both sides then wipe down (firmly) with a damp sponge from the inside of the wall toward the seam (on both sides) then up and down to remove excess glue.
To fix bubbles, make a small cut in the bubble with a razor knife. Insert the end of a glue applicator in the slit and squeeze in a little adhesive. Wipe away excessive adhesive with a damp sponge and press the paper against the surface to force out the air, using a plastic straightedge.