Those hairline wall cracks that appear above windows and doors are caused when framing lumber dries out and shrinks or the structure settles. Although the fractures are small, they're persistent: Simply spackling over the trouble spots seldom hides them for long.
For a more permanent repair, scratch along the crack with the pointed tip of a can opener. Hold the tip at a slight angle to undercut and widen the crack. Bridge it with strips of adhesive-backed fiber mesh drywall tape. Then spread a thin coat of joint compound over the taped crack with a 4-inch-wide putty knife. Lightly sand the compound after it's dry, then apply at least two more coats of joint compound, feathering each one slightly to blend it into the surface of the wall.