How do you do this: How to cut perfect fit crown molding?
Cut an 8-in. piece of the molding you're putting up, and staple a piece of medium-grit sandpaper over the top edge. Then hold the molding in place against the wall and ceiling exactly as it would be attached, and slide it back and forth to sand the area smooth. Not only do you get a perfect fit, but the slight ridge formed by the sanding helps hold the molding in position when you nail it.