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.
Great tutorials for DIY molding and trims. Perfect Trim on Doors, Windows and Base Moldings. Tricks for getting tight-fitting joints on door and window casings and on base moldings.
I highly recommend that you make yourself a full set of eight crown molding templates, label them and use them as a guide to prevent cutting.
Base board Installation - Temecula, Murrieta, French Valley, etc