Fine Homebuilding, January 2010 has an article for fixing damaged plaster walls that is pretty comprehensive

I've done some of this working on an 1860's wood frame townhouse-

When I took down walls, I used a circular saw set for the depth of the plaster, and cut the plaster into squares. This way the plaster can be removed and then you can remove the lath (if its wood) so you can save it to fix holes. Dont just go at it with a sledge, as you will break plaster keys elsewhere and end up with loose plaster on the walls and ceiling

Where the walls meet (the stub) there will be no lath, and this is where after removing the corner studs you can use the old lath to fur out the new gyp. board. Build up to the plaster surface with gyp. board so the mud is minimal. Paint the edges of the plaster with Plaster Weld- it is pepto bismol pink and acts as a primer to the plaster so the plaster does not suck all the moisture out of the mud. Use fiberglass mesh to bridge over the whole area and skim coat.

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