How do you hang pictures from the trim of older houses?

I live in a Victorian style home built in 1900. The walls are made of horsehair plaster and I know that those don't take too well to nails... but that the trim has a lip designed for hanging pictures like in the olden days. However, with time this has become less and less, and I would like to hang a mirror above the fireplace nail-free and I have wire, but I'm not sure what other piece of hardware I would need in order to use this technique.