The real key to a good plaster repair is to get the new to bond well with the old.........
Scrape any loose or broken plaster from the crack and surface.........use a pointed tool to open the crack to get a good clean repair...........
Get a gallon of Latex Ad-Mix (located in the Home Store next to the plaster) to "glue" the new/old work............Take a brush and paint all exposed surfaces you plan to plaster with a full strength mix.......this keeps the old from sucking the moisture from the new work and gives flexibility to the repair......
Mix your patching plaster with the latex-Ad-Mix........and apply in layers to fill the voids...........Try finishing/blending the finish coat with a large damp sponge to match the existing texture of the wall.........Easy Huh?......doug