This problem was brought back to me by a recent blog article by Emma Darwin:
In this article she pretty well sums up the problem. She provides a link to a good article on the proper use and the misuse of prologues. She also wrote an article back in 2011 that deals with some problems and their solutions when backstory is needed.
Many new authors seem to think that they simply must spill everything for the reader right away, and they believe that the reader won’t understand the story without all the background detail. While these details may seem fascinating to the author, they’re often far less so to the reader. At the very least, backstory stops the present story in its tracks. Consider this following opening to illustrate what I mean.
To read all of Rick's article about Flashback, click here