The biggest mistake made when hanging art is not matching the size/shape of the art work to the space you are hanging it. Work should look "comfortable" in its space not cramped or looking like it is awash in a sea of wall.
|Layout follows vertical space|
|Long console needs substantial work to fill space|
|Perfect size and composition for top of stairs|
|Size and shape works with width of headboard|
|Art shape and size follows wall shape|
And there you have it. Large works for large spaces or smaller works massed together for impact. No little pieces lonely on a big wall. No large pieces squat into a small space. Oh, did I mention I have a lot of opinions on gallery walls? I'll save that for another tip.