Picture Hanging Jig - Updated
You need a picture hanging jig. To locate the exact spot for the nail to hang a frame find a strip of wood longer than the distance from the cord to the top of the frame. Drive a short brad or screw into the end of the strip so the point protrudes just a bit, hang the picture on the head of the brad/screw and holding the strip manoeuvre the picture to where you want to hang it, press the strip against the wall, the point makes a mark on the wall. Use the mark as the reference point for hanging your frame.
The beauty of this technique is if you are hanging multiple frames any variation in cord tension or position of hanger on the frame will be avoided. Just align to the top, bottom or centre line of the frame.
I now use this method of hanging, it avoids trial and error and is so much quicker than all the other methods I have tried.
I am grateful to www.sawdust.com for the original idea. Update: the link no longer works, similar info available on Homestoriesatoz.com