Everyone I know used called it something different but when I came up with the term "power clashing" it really stuck. Print mixing, as the New York Times called it, has been everywhere. I can't tell you the rules, just go with it. Whether its stripes, floral or otherwise, use you own judgement. Here are some examples.
Peter Pilatto spring 2013
Man Repeller aka Leandra Medine in a Peter Pilatto skirt and gingham button up.
Etro Spring 2013
Etro resort 2013
Missoni resort 2013
Cavalli resort 2013
Me with Cesar Galindo and his pant. Well, pants he makes, not pants he wears...assuming here...