Consider the entire room when hanging pictures.  The Roller Coaster Effect should be avoided at all costs.  The eye should not have to travel up and down and up and down as it scans a space. 

Place items at the same level by aligning the tops of frames even when they are housed on separate walls.  When critiquing your space, imagine a line like the arrow in the photos. Better yet, get out your tape measure and verify the distances from the ceiling to the tops of all the frames are all the same.

In some cases, large vertical pieces of furniture must be considered as well.  In my own home, I have a large wine rack, book cases, a huge standing mirror, and art along the walls of the Living Room.  In order to align them along the same plane, I had to measure everything and adjust some only a matter of inches.  It was worth the effort.  We save our thrill rides for King’s Dominion.