First. You can get the plaster washers from Charles Street Supply in Boston.
These washers are 1 inch in diameter and require the use of a screw to attach to the studs and wooden lath.
When you attach the washers you need to be sure that they are pulled up tight against the wall/ceiling. This way it is easier to apply a skim coat over them.
You need to start at the studs and follow them to the point where the looseness ends. Once the studs are taken care of you then go throught the middle of the studs to catch the laths.
If the plaster is very loose you need to start with a prop. Use a 2x4 to prop up a piece of plywood against the wall or ceiling then start from the outside of the loose area and work your way in to the middle. Once you get towards the middle you may find that the plaster is beginning to kink. Use a blade to break a little plaster loose so that the plaster will not break off. (Sometimes it is better to start in the middle, you will need to judge by the way the bulge is whether you will need to start in the middle or from the outside).
Once the plaster has been re-attached you then need to apply a bonder to the area. Use a PVA based bonder and skim it with EasySand90. It may take three or four coats. Sand when dry. Prime with a good high quality OIL based primer and paint.
If you want to see the method for re-inforcing flat plaster using these plaster washers please follow this URL: http://www.ornamentalplaster.com/slide_shows/Ornamental_SlideShow/tinneyhouse/index.htm