I came across this new product that is great for hanging pictures. Its called the wallmarker and is an arrow-shaped piece of chalk that sticks to the back of the picture you are going to hang (its also perfect for those dreaded dual brackets) then hold the picture up to the wall where you want it hung and just push a little. The chalk leaves a mark pinpointing where to put the nail.
They come in packs of two, a blue arrow for light colored walls and a yellow one for dark walls. It's also priced good to, only $2.99. You can get them at this website- www.wallmarker.net