If the plaster has metal lath, a hacksaw blade is the way to go. It's helpful to use the blade so the teeth cut on the push stroke. You can get a hacksaw-blade holder that allows using the blade like a keyhole saw.
A hacksaw blade can be used for wood lath, too. The best way to loosen as little of the lath as possible is to make the vertical cut farther from the nearest stud first, then the cut closer to the stud; the closer, the better.