Cut the pieces 1 degree off so the inside or facing corner touches first when the two pieces of molding come together. This used to be standard practice years before chop saws were around. This ensures that the corner is always tight and gives it play. Inotherwords, don't make the cuts parallel to eachother. I just did my kitchen crown molding this past summer and it came out flawless. Of course a nail gun and compressor is mandatory