Mike, put crown molding upside-down in your miter box. It's that simple. If you are having problems with mitered inside corners, the problem is with your walls and you will have a lot better luck with a coped joint than a mitered joint.
One neat way to do it is to use a framing square from the back of your molding and find the dimensions of your molding, measuring as if the framing square was your walls. Then subtract a quarter of an inch from both dimensions and rip a 2x4 to the smaller dimensions. Install the spacer blocks you just cut in the same orientation to the ceiling/wall junction, then put the moldings on over that. Makes nailing a lot easier and you don't have to worry about the smiley faces you sometimes get when when you just nail the crown molding up.