I once heard literary agent Donald Maass give a workshop on his Writing the Breakout Novel. One thing he said I will always remember -- backstory is called backstory because it belongs in the back of the story.
Too many beginning writers make one big mistake -- loading their first scene, first chapter, first quarter of the book with way too much backstory. They feel that the reader won't understand their protagonist, their plot, their world, unless they TELL ALL upfront. However, this usually deadens the forward movement, the energy of the story, and leaves the reader without any urgent mystery to propel them onward.
One of the most important mysteries you should be pushing your reader to discover is the compelling backstory you’ve withheld. Think about JKR. Her masterful withholding of backstory -- what actually happened in Godric's Hollow, which side was Snape truly on, and why did Voldemort want to kill a one-year-old baby -- is the energy that thrust the reader not only through the first book, but the next several to come as well.
One huge reason Harry Potter was such a phenomenal success was due to JKR's witholding of backstory. She did not release it until her readers were beyond dying to know!
When unloading backstory, at least in the beginning of your story, less is more. Withhold as much as possible without leaving your reader confused. Explain just enough to have the current action make sense but to keep a question pushing your reader onward. Then, dribble the backstory in as necessary, in bits and pieces, carefully woven in. Preferably, release enticing nougats in dramatic ways and only after the reader's curiosity is at a fever pitch.
For some specific examples of what backstory JKR used and what she withheld, I'm working on an upcoming post that breaks apart the first chapter of Sorcerer's/Philosopher's Stone with all its strengths and weaknesses. But for now, have you had to revise out too much backstory in one of your manuscripts? How did you do it?