Really the "right" way to do the inside corners of the crown molding is by coping the end of one of the strips of molding, like Bitt mentioned. Coping rather than just cutting a miter will give you a MUCH tighter connection in the corner, and it will not pull away/separate over time. Really, coping is simple. It just involves butting one of the strips up against the wall, and then mitering the second strip then cutting along the contour of the molding at an angle perpendicular to the miter cut. Here's an example.

Really, I think coping is an easier way to make it look great - but if you've never done it before, it takes a little practice. The tools involved to do it right are a miter saw, coping saw, and a file that can get into the contours of the molding.

- trs