Past perfect is one of those difficult tenses. Sorry to bore you all, but today I'm in the mood for a grammar rant. (Wearing the grammar teacher and editor hats today.) I love tenses. I love all their different names and the way they interconnect, and how you use them differently. For the uninitiated, past perfect is a tense formed with "had" + past participle of a verb, eg had walked, had danced, had brought, had eaten, had ridden etc.
Past perfect is used for actions completed in the past before another action in the past. It's like a second step back in time. For example, if I'm writing in present tense and want to indicate a past event, I use simple past: Today I go to the milk bar; yesterday I went to the bakery. But if I'm writing in past tense, then I can't just use simple past, and need to show another step backwards: Yesterday I went to the milk bar; the day before I had gone to the bakery. Should be simple, right?
I remember someone saying to me when I first started taking writing seriously that you should never use "had had", that the second had was never needed. I know now that this was a stupid piece of advice. Even then it felt wrong, and so when I wrote my fledgling stories I would contract the first one and use I'd or she'd or whatever, just because I could tell instinctively that I really did need both verbs, but people tended to let me keep the two verbs. Perhaps where this advice stemmed from -- or at least should have stemmed from -- is that when you segue into past perfect, which indicates a second step back in time, you only have to use the had the first one or two times and then can slip into simple past tense. Simple past is much more direct. Too many "hads" bog down a sentence. Pace stagnates when it should be racing. Of course the trick is to signal clearly to the reader when you are returning to the "now" of the story, whether it's in present or past tense.
But the problem I see often in student work is use of past perfect when simple past will do. This is usually because a student is writing in present tense and so should use simple past for past events, but I think they're so used to writing in past tense that they take that second step backwards when only one is necessary. They'll also use past perfect when they really want present perfect (have gone, have fished, have eaten, have drunk). Obviously for some it's tricky stuff. I'm lucky because it's always come naturally to me. Sometimes I think it only affects them on paper -- that they can handle tenses properly in speech, but this isn't always the case. My tip, as with any problem when writing, to improve your tenses? Easy: read lots. Read widely. And pay attention to what the author is doing.