In the last two manuscripts I've edited (one a very good novel, the other an interesting memoir), both writers have struggled to work well with back story. As you probably know, back story is the story or pivotal events of the characters (usually the main ones) before the novel begins or the specific event in the memoir took place.
There are some good rules for back story that I use in my own fiction writing and encourage my clients to use, although my encouragement is not always successful as some people are married to a particular memory in a particular place. But here are some problems and some suggestions.
Tip #1: Use back story sparingly and at just the right moment.
In the novel I was editing, there were some important pieces of information in the growing up of one of the major characters that needed to be inserted somewhere along the narrative. The author chose to do these as memories sparked in the mind of the narrator. Unfortunately, there was no pattern to these memories and they were not linked to external events, just a kind of loose "that made me think of Patty when..." As the reader, I found myself unable to figure out how old the girl was in each memory and how the memories fit together to help me understand her. I suggested the author go back and put the memories in chronological order and do at least some of them as flashbacks, rather than just the narrator telling us about them.
In the memoir I was editing, events that had been timely in earlier chapters showed up in later chapters as back story. Now, it's fine to allude to earlier information. Authors do that all the time. But information can't be presented as new information in several places. That tells the reader that the author has no control over his information. This is a self-editing task.
Tip#2 Don't insert a flashback or back story into an action scene.
Good writers don't have characters stop and remember something in the middle of an argument. An exception is something that passes quickly through the mind of the character. "When he shouted at me, I heard my dad's voice yelling at my mother." That much we'll buy. But a whole scene from childhood will throw the reader back in time and diminish the drama of the scene at hand.