When you hang heavy items, like large mirrors or framed art, you want enough support to keep it firmly on the wall. Using two hooks is the best way to do it, but hanging those hooks first and the art second can be tricky. Use a piece of scrap wood to make sure the hooks are the right distance apart. Here's how. You need a scrap of wood that has a straight edge on one side and is 15 or more centimetres long so that you have a solid amount of space between the hooks. Mark the centre of the wood, then place it under the wire attached to the back of your frame. Measure the distance from the centre mark on the wood to the top of the frame.
Mark on the wall where you want the top of the frame to hit, then use the distance you just measured to mark where the hooks should sit in the wall. Hold the piece of wood so that it is level and lines up with this mark, then mark where the ends of the wood meet the wall. Nail a hook at each of these marks, then hang your frame. For step-by-step visual instructions, check out the video above starting at 02:15.
How to Hang a Heavy Mirror [This Old House (YouTube)]