There are several different methods for hanging a mirror on a wall. The best method depends on the type, size and weight of the mirror, as well as the type of wall surface and whether you can anchor into studs or not. For safety (and to avoid seven years of bad luck, in the event of a breakage), it's important to hang a mirror securely, using hardware that's more than strong enough for the job. The two main types of mirrors are framed and frameless. Framed mirrors are hung much like framed pictures---with wires or hanging rings attached to the frame and screws or hanger hooks attached to the wall. It's best to fasten wall hangers into a stud, whenever possible; if not, use a suitable hollow-wall anchor, such as a self-drilling drywall anchor or a toggle bolt. Don't use the plastic, expanding-cone anchors that come with some hangers; they don't hold well enough. Frameless mirrors typically are installed with metal channels (for large plate-glass mirrors) or plastic clips (for smaller, decorative mirrors), using the basic processes described here.
Position the mirror on the wall in the desired location. Check it with a level to make sure it is plumb and level (with a round or oval mirror, you'll just have to check it with your eye).
Make pencil marks along the top and bottom edges of the mirror. Also make small marks at each side edge, near the corners. Set the mirror aside.
Measuring in from the side marks, mark the location for each mirror clip, spacing the outer clips a few inches from the sides and any remaining clips evenly in between. Mark for clips along the top and bottom of the mirror.
Drive a small finish nail at each clip marking. If you hit a stud, drill a pilot hole through the drywall and into the stud, sized for the clip screw. If there is no stud, install a hollow-wall anchor centred on the mark, following the manufacturer's directions.
Install the clips for the bottom of the mirror, using the provided screws. Set the mirror into the clips, then install the clips along the top of the mirror.
Mark a level pencil line on the wall representing the bottom edge of the mirror. Use a stud finder or a small nail to locate all of the wall studs along the pencil line.
Cut two pieces of J-channel to length, matching or nearly matching the mirror's width, using a hacksaw. If necessary, drill a hole through the rear flange of the bottom channel to match each stud location. Install the bottom channel on the pencil line, using the manufacturer's recommended screws.
Measure up from the bottom channel and mark a level line for positioning the top channel. Allow for just enough clearance to slip the top edge of the mirror into the top channel then slide the mirror down into the bottom channel, capturing both edges in the channels.
Install the top channel on the marked line.
Apply mirror/glass adhesive sparingly to the back side of the mirror, using a caulk gun. Insert the top edge of the mirror fully into the top channel, check the side-to-side position of the mirror, then bring it flush against the wall and let it slide down into the bottom channel. Check the mirror's position again, and make any sideways adjustments before the adhesive sets.
Large plate glass mirrors can be very heavy. Make sure to have adequate assistance for any mirror installation, or consider having a professional install the mirror.