Hi Mark The way I do it is to rip a triangular 1 1/2 by 1 3/4 piece from a long 2X4 and nail this up into the corner where the wall meets the ceiling. then just center nail the crown molding to this. It will draw in the top and the bottom of the molding evenly.
To prevent splitting when nailing the ends, predrill using your nail as a drill bit chucked into your cordless drill. You can dull the nail tip first before nailing, but I've found predrilling easier and a bit more secure.