Here are a few things that help me a lot.
First, try to keep the opening on a straight part. That will make it easier to get the seam to lie flat.
Second, iron in the seam allowance before you sew. Now the seam knows where to be when you get there to sew it.
Third, stay stitching on the seam line. As someone else said, that keeps it from stretching while turning.
When the seam is ready to be sewn, I like to top-stitch as close to the edge as I can. That edge doesn't even go under the presser foot. I make sure I can see the very edge of the bottom piece as I sew on the top piece. They are almost even, but I need to see the bottom to make sure they are right. For certain applications, a very tiny zig-zag works here, too.