Don't go into the backstory too soon. It is understandable that beginning writers are tempted to do so. The writer wants the reader to empathize with his characters. The writer knows his characters' histories and wants to share this with the reader. The reader is of course curious about the past. But that interest will be urgent only when the present story has sufficient tension. The writer must ensure the story is moving forward more than it is moving backward. The writer must trust the present story. The writer must also trust the reader, who doesn't care if the characters aren't fully explained. A little mystery is a good thing.
Use your backstory to complicate the present story, not explain it. Good fiction, good drama, is about action, not explanation. The writer's job is to portray action and let the reader draw her own conclusions. For example, if the present story concerns a mother struggling to be patient with her troubled children, and the backstory shows that in the past she often has not been patient, the tension level of the story goes up.
Discover something unresolved in your characters' past. The skillful writer constructs the story to force her characters to deal with their ghosts. The story is a shovel, so to speak, uncovering the buried past. The backstory should come up, not when it is convenient to the writer, but when it is necessary to the story. For example, if a brother and sister are arguing about what to do with the family home after their mother has died, it is only then, and not before, that the writer should delve fully into their past conflict.
Keep in mind the difference between backstory and flashback. Flashback --- a technique mostly associated with film --- is a sudden and brief jump back in time. In fiction, this usually involves a character experiencing a quick flash of memory, and then returning to the present. Backstory is different, and should, like the present story, be built around scenes. A scene is a kind of mini-story --- with its own beginning, middle, and end --- and is the building block of most good stories. The writer must pick his spots. But when he chooses to go into the backstory, he must commit fully, and deliver vivid and dramatic scenes that are every bit as compelling as those in the present story.