When I took sewing lessons several years ago, my teacher was especially excellent at showing me little tiny details that make for a more finished / professional look of a garment. One thing she taught me that I haven't really used (or thought I needed to use) until recently was the trimming of the inside seams when using a facing. At the time, I didn't really understand the reasoning, but with all the Oliver + S patterns I've sewn recently, this little trick has come in quite handy. The only time I've ever seen this technique illustrated was in the old sewing books.
When turning a facing seam, especially around collars, necklines, underarm sleeves and even hems, it is always necessary to trim the seams on the inside before turning. First trim both seam edges to 1/4 inch, then trim the FACING to 1/8 inch giving it a layered look. If the seam is especially curvy, I still clip the curves. When you turn it right side out, the facing (inside piece) will turn itself just ever-so-slightly more to the inside so that it isn't really seen from the outside. If your facing piece also includes interfacing - then you will triple-layer the trimming, cutting one just inside the other.