Okay, so maybe you already know this, if so, just ignore this post. I can't remember where I first heard about this method myself, but I thought I'd share for those of you who are as of yet uniformed. This is especially helpful when hanging groups of art together.
1) Measure the size of the frames you want to hang and cut pieces of craft paper to the exact size of the frames.
2) Using masking tape, place the cut pieces of craft paper on the wall in the location you want to hang the pictures. Then adjust until you have the exact spot where you want to place the wall art.
This step is great because you can play around with the wall position of the various pieces of art until you find the perfect spot and can actually see exactly what it will look like before you've hung a single picture.