The first rule of backstory... Only tell the reader what they need to know in that moment to understand the story. It isn't necessary to load a reader up with information that they don't need right away. Does the reader really need to know that the main character's father has been in a mental hospital for the last six years? Or that the main character's favorite team is the Red Sox? Or maybe the main character likes to wear cut off jeans and holey T-shirts every Saturday when he (or she) likes to go to their favorite local pub. Unless that information is necessary to the story right in that moment of the story, it should be come at a later time.
Okay, so the time has come when there is some important information that must be put in the story. What then? Then, it should be dribbled in little bits at a time; A little here and a little there can go a long way. Too much information in one place will pull the reader from the story, and may make them stop reading altogether. These large clumps of backstory are often called "info dumps" and should be avoided at all cost. How can this be avoided?
Here are a few ways to add backstory without hitting the reader over the head with the information...
- through dialogue
- through narration
- through "inner" dialogue
Make sure when using these techniques that it isn't being spoon fed to the reader. Spread it out to make it easier to swallow and your reader will love you for it.