This is killing me. I'm writing in the past tense (third person omniscient). A few times, I've found myself jumping back and telling about something that had happened before the time in the storyline. I have four scenes which are presented by jumping back from the main story line. "Oh, by the way, this had happened." Two of them are unavoidable: the story does not start with the characrters' early childhoods. Two others are within the time frame of the main story. It is these two that bother me: they are essential to the plot, but to me it is obvious that by inserting them in someting else I am saving on developing them on their own. They come out somewhat shorter with only the essential information and a few emotional touches. Brief sketches instead of fully developed illustrations which create the right impression. Are there any examples in accepted literature where this has been done with success? Or is there a rule that says "this is almost always bad." Getting the tenses right is a separate story which doesn't really belong in this thread.
Flashbacks and such are done frequently, but they have to be done right. It stops the pacing of the current story, so doing it too much can create a traffic jam of plot and story within the main narrative. I don't have any famous or well known examples to cite for you, but I'd recommend doing what's right for the story. If these two flashbacks are bothering you because they're not that well developed, then develop them. If you find, by developing them, that they take up too much time from the main narrative, consider splitting them up, if it's possible. You've got a decent number of options available to you; I'd recommend that you explore them a bit.
Jodi Picoult seems to do this alot, atleast in the novels I have read from her. She will go back in time and write something that is important to the story. In 19 Minutes, the story jumps back and forth from the present to the past explaining how Peter ended up shooting students at the school. Other characters are explored during these flashbacks too. In The Pact she does something similar. The present is about the shooting and the aftermath with the trial. While the flashbacks explore Chris and Emily's relationship from a young age up until the night of the shooting. There is nothing wrong with flashbacks and or going back in time to explore the characters further. It gives the reader a wider view of whats happening within your story. Its all in the execution. If you do it right it will only add to the story. Do it wrong and it becomes a mess.