First step is to identify the type of damage your walls suffer. The treatment may differ somewhat for cracks and loose plaster scale. Once you identify the cause and type of damage the wall plaster suffers, you can prepare the surface. Remove the lose plaster first. If a large chunk of plaster has lost grip from the lath behind it, remove the plaster and form rectangular patches before repair. Use repair mortar to cover the patches completely. You can also make use of wall tiles to cover the wall and avoid plaster damages anytime soon.