" One of the general patterns of good (i.e., striking and memorable) writing is the effect of repetition. If you use a certain element—a plot device, an image, a noticeable phrase—once, readers may or may not notice it consciously, but it doesn’t disturb the flow of their reading. If you use that element twice, they won’t notice it consciously—but they will notice it subconsciously, and it will add to the resonance of the writing or to their sense of depth and involvement (and if it’s a plot device, it will heighten the dramatic tension). But if you use that element three times, everybody will notice it the third time you do it. "

― Diana Gabaldon , "I Give You My Body . . .": How I Write Sex Scenes