Plaster is very hard to work, I know because I have done alot of plaster wall, I have found the the best way is to use painter taps mark out the box and get a new sheetrockers jab saw drill the corners and cut slow. if possible I try to cut the plaster first then go after the lath (if Wood) you may run into rock lath which is an early version of sheet rock. Most of the damage comes when trying to snake the wire. The best method for this is Jack chain with a weighted end(if coming top down) if going bottom up use a snake with a smaller piece to reteive. Never attempt to put your hand in the hole this could be very damaging to the plaster.Also look @ wall fromside priop tu cutting you can ussually tell where the plaster is loose and try to avoid cutting there and tell the home owner up front that there is the potential that you will have to possibly repair wall @ an added cost.
