Decals that remind visitors of the awesomeness of certain anime characters or superheroes often adorn the bedrooms of children, but when a decal starts peeling off prematurely, it can make a bedroom look messier than it really is. The loss of adhesion can be due to humidity, greasy walls, glossy paint or all three. It may also be the result of inferior adhesive on the decal. You'll probably need fresh adhesive to re-stick the decal, but since you might want to take it off someday, don't use permanent glue such as contact cement or superglue.
Pull the decal off the wall. If it's hard to remove, warm the decal with a hair dryer to soften the adhesive and work it off carefully with a plastic putty knife.
Flatten the decal by putting it between two pieces of waxed paper, setting it on a table and placing one or two heavy books on top of it. Leave it for a day or two.
Clean the wall with a solution of detergent and water to remove grease. If the wall is painted with glossy paint, lightly sand the area on which you want to put the decal to etch the paint, using 220-grit sandpaper.
Remove the wax paper covering the sticky side of the decal and spray multipurpose adhesive on that side of the decal. Pick up the decal carefully and press the sticky side against the wall.
Keep pressing the decal against the wall for about a minute, using a plastic putty knife to put extra pressure on the edges.