Hold crown molding in position...

woodworker.com: Mitermaster holds crown molding for faster cuts. With the jig you can accurately cut inside or outside corners of crown molding with fewer mistakes. Slide molding into holder, place the back of holder against fence and cut. 11-7/8" long aluminum holder keeps molding in proper cutting position and helps prevent slipping or shifting. Three holders to accommodate 2-1/4', 3-1/4' or 4-1/4' wide crown molding cost about $22.

Rockler has a crown moulding jig for $20. Jig holds molding at correct angle to eliminate complex compound angles. All you need to do is make a 45º cut.