A mirror can be very heavy -- so heavy that it takes two people to hang it. A regular picture hanger cannot support that much weight; neither can a nail nor even a screw if installed in the wrong place. The best hanger for a heavy mirror is a 3-inch screw sunk at least 1 1/2 inches into a wall stud, but studs aren't always where you need them. A good alternative is a toggle bolt attached to the drywall -- two of them if the mirror is exceptionally heavy.
Determine the best lateral position for the mirror, using a tape measure to measure distance and a pencil to make a light line on the wall at the approximate height of the base of the mirror. The best position for a mirror is often at the mid-point of the wall, but you may want to offset it to accommodate a painting or piece of furniture.
Drive a 2-inch screw through a 2-foot piece of 2-by-4 lumber so that the tip of the screw emerges about 1/4 inch from the back of the board and there is enough space between the screw head and the board to hold the mirror wire or hanger. The screw should be about 2 inches from one end of the board.
Hook the screw head into the mirror wire or hanger in such a way that the other end of the board extends upward from the top of the mirror. Get a helper to hoist the mirror into its approximate final placement while you hold the board. Maintain upward pressure, as if you were holding the mirror up all by yourself.
Push the mirror against the wall when you find the proper height. The screw tip will make a mark at the exact point at which you need to install the screw or toggle bolt to hold it to the wall.
Tap the wall behind the mark made by the screw to determine if a stud is there. You can also use a stud finder. If you find a stud, drive a 3-inch screw into it, leaving the head about 1/4 inch above the drywall.
Drill a hole for a toggle bolt if there is no stud, using a drill and drill bit. The size of the hole depends on the size of the toggle bolt you use. Tap the toggle bolt sleeve into the hole with a hammer, then screw in the bolt with a screwdriver, leaving the head 1/4 inch above the drywall.
Measure the distance from the hole to the outside edges of the mirror to install two toggle bolts if the mirror is heavy. Most toggle bolts can safely support about 75 pounds, so if your mirror weighs close to that, you need two of them.
Place a level on the mark made by the screw, center the bubble, and draw a line along the edge of the level to points halfway to the edge of the mirror on either side of the mark. Make new marks on these points and install a toggle bolt at each mark.
Lift the mirror into place and hook the wire or hanger onto the screws or toggle bolts you installed in the wall. Let go of the mirror slowly, and straighten it once you're sure it's stable.