hanging a mirror in lath and plaster

I could use some help in hanging a 20# mirror on a lath and plaster wall. It seemed easy, as there is a hook on the back of the mirror and I figured I would just hammer a picture hook into the wall, hook the mirror over and I'd be set. However, there is no stud where the picture hook needs to go, and the nail of the picture hook has hit the lath and plaster and now I have a 1/2" hole with the lath showing!!
No problem to patch the plaster and start again, but how to prevent this next time? Use something other than a picture hook? Drill first? other ideas?
many thanks

 

A 20lb mirror is really not much weight to speak of. If the plaster is sound a mirror hanger should do it. If not, nail a piece of 1/4" plywood to the wall and then nail the hanger through that.

 

Use a molly.

Patch over the exposed lath and then drill into it after it dries. Push the molly through the hole and tighten the screw so that the molly expands inside the wall. Unscrew slightly and hang picture from that.