Simple Backstory: Most DMs and designers hate to hear it, but much of the time lavished on history and background is wasted energy. Players never find out who dug the tomb, how the wizard was betrayed by her apprentice, or why the assassin guild changed sides and disappeared. Working on backstory doesn't improve the gameplay experience for anyone but the bards and scholars obsessed with legends or lore. Unless it connects directly to action in the current timeframe (and the PCs have a way of learning it), skip the involved history. Save that for sourcebooks.
This is not to say cut it all. Details of which faction can be turned against another, which guard might take a bribe, or what the villain ultimately plans to do if the party doesn't stop him are all appropriate. Make sure your backstory is recent and relevant; avoid anything that starts "Thousands of years ago..."