When you have a crack in a plaster wall, if you want your repair to last you need to put fiberglass tape over the crack before plastering it up.
Brian in Niagara Falls, Ontario has a great idea for patching a series of cracks. He puts a spray glue over the area, then pushes window screen into the glue. Then he follows with the drywall compound over the screen. This locks the cracks under the screen and gives a good surface for painting that will not crack again.