Persona-Driven Development: Meet Customers First, Write Unit Tests Later
krisjordan.com
krisjordan.com
http://www.boxesandarrows.com/view/interactive http://www.krisjordan.com/2008/09/07/10-minute-mock-prototyp... http://www.istartedsomething.com/20071018/powerpoint-prototy...
There are tools specific to GUI-prototyping that have other upsides/downsides, a great list is here: