Start where the protagonist's problem starts, or just before that, and feed in the backstory later. Be especially wary of books that start with the protagonist on a journey, thinking about what awaits her at the destination.
Editors frequently mention that as an example of a boring opening.
Make the ending a conclusive one, reinforcing the themes of the book and the progress of the protagonist. So when you're starting the middle, think of how the protagonist can be challenged. That is, they exist to make things happen in the plot, but their own conflicts and issues shouldn't be part of the story. But in most protagonist-centered popular fiction, tracking the secondary characters' lives and loves is going to waste time and confuse the reader. Watch out for long passages in a secondary character's viewpoint which dwell on his problems and not on the protagonist.