When you're writing a story in simple past tense (had), and you want to use past perfect (had had), what guidelines do you follow? For example, say that you begin a story in medias res in simple past tense, and then you want to establish some backstory before returning to the current scene--when you switch to past perfect for those previous events, do you strictly maintain it even if the passage of writing goes on for several pages, or do you at some point switch to simple past for previous events, on the assumption that the reader will intuitively be able to shift gears as needed? Do you edit yourself if you catch yourself switching tenses like that? Or do it consciously and try to be unobtrusive?
If you're writing a story at length that alternates between two timelines, do you use the verb tenses as a way to demarcate each section, or is that consideration irrelevant to how you want to structure the story?
How comfortable are you with a writer switching from simple past to past perfect--what kind of guidelines do you like to see maintained? What throws you out of a story?