Taken from http://www.stretcher.com/stories/990308m.cfm:
"Plaster is a difficult material to hang stuff on. I have found plaster walls to vary dramatically in their ability to hold picture hangers. However, using #8 or #10 (at least a couple of inches long) hexhead screws through the plaster and into the lathe will give you great strength in most plaster walls. Of course, the strongest method is to install the screws into the wall studs, though the studs are seldom in the right place. A subcategory of Murphy's Law, I think.
Start the screw hole with a drill bit smaller than the screw. Don't drill through the lathe, just start the hole so the screw grabs in the plaster. You can use a masonry bit, but I have found that regular bits will work in most plaster walls but will become dull. A regular bit will drill a cleaner hole than a masonry bit.
You must test the screws for strength. Use a hammer or pair of pliers and exert steady downward force on the screw head, at least equal to the weight of the mirror. If the screw seems firmly attached, you are probably all right. Subjective judgement is all you have here!
I always put a small metal washer on the screw before attaching it to the wall. Place the mirror's hanging wire between the washer and the wall. This will prevent the wire from "jumping" off the screw head. With a heavy object, it is sometimes difficult to get the wire between the washer and the wall... especially when working solo! When this happens... and if getting a helper is impossible... stabilize the washer using a small amount of hot melt glue, Goop, or adhesive caulk between the washer and the screw head, and let it dry. In a pinch, you could even use some of that tacky gum... I think it's called Fun Tack... that kids often use to hold papers and other light objects to their walls.
If the plaster/lathe does not hold the screws well, you may have to install a strip of 1x3 1" pine or plywood between studs that will be hidden behind the mirror. Attach your hanging screws to the wood strip to locate the mirror. "