There is a product called the Magic Miter that is fantastic. My wife purchased one for me last year because I am installing crown moulding in the basement. I'm normally not a "gadget" person, but the miter is a great idea. It uses a corner template to measure the angle of each corner, inside and outside, and then transfer that angle to the miter box. It also comes with a great set of instructions for cutting and installing the moulding.