The secret for a glove-tight fit for trim corners is a coped joint. With this technique you can even make complex crown moldings fit without leaving gaps. http://www.familyhandyman.com/carpentry/trim-carpentry/how-to-cope-joints/view-all
This handy little “cheat sheet” can be used on any crown molding project that requires inside and outside corners. Visit Dunndiy.com to learn more about how to measure, cut, and install crown moulding.