According to my interpretation of a video by Peggy Sagers from Silhouette Patterns, buttonholes in blouses should always be VERTICAL. Since I have almost always made mine horizontal, I had to figure out a way to quit violating this buttonhole law.
Hence, the video.
And if you prefer not to listen to me, here is a drawing I made for you of what I DO to keep the buttons from sliding up and down and the hemline and top edges becoming uneven. And REMEMBER, this is just what I do. I didn't learn it from anyone else, and someone else may have a much better idea I haven't heard of yet.