The first poster my wife got for me after we were married was a Mondrian print. I love how he created such pleasing images from such simple forms. Now there is a web app for creating your own Mondrian style creations.
http://www.compositionwithjavascript.com/
What really struck me was how quickly things go off the rails. It only takes a few clicks before a balanced harmonious composition gets too busy or just plain ugly.