Here's one good rule of thumb for your first chapter: No flashbacks. Why? Did some tired, cranky editor make up an arbitrary rule?
No. In a recent lecture on plot, Carol Lynch Williams gave these reasons why flashbacks should be used rarely.
If a flashback is needed in the first chapter, it’s a good indication the writer has started in wrong place. A story should begin when something in the character’s life has changed, or is about to change.
A good story is a rising progression of action. Flashbacks almost always slow the movement of the story.
If you have to use a flashback, and Carol said she did find them necessary in her award-winning book, The Chosen One, keep them short and simple. Get in and out of each flashback quickly. Experiment with techniques, such as alternating past and present tense, to let the reader know where they are in time.
So editors have good reasons for asking writers to be wary of flashbacks, especially in the beginning of your book. Use flashbacks sparingly and well. It may be a good way to make sure your editor doesn't end up cranky.