One of the hardest parts (for me, anyway, and since this is my chance to talk I’ll stick with it) of constructing a great story is the idea of flow. It involves the transition from one scene to the next that takes the reader from humble beginning to ultimate, climactic end.
At the heart of this idea is the need to not waste a scene. Each scene (each word, really) in a great story serves a purpose: developing characters, driving the plot, raising the stakes, etc. Little side adventures, which seem like great fun for me as a writer, ultimately hurt the overarching story.
Take the first Harry Potter book. What appear at first to be fun side adventures for Harry and the gang—the midnight duel, rescuing a dragon—are actually all the same story, in disguise: the ultimate revelation and struggle against Voldemort’s secret agent. Harry needs a reason to go through that third floor corridor and see what’s there, and he needs a reason to be in the Forbidden Forest after dark to encounter the unicorn. The midnight duel and the dragon serve these purposes . . . while also being great fun.
Mystery writers like Agatha Christie are great at this, I find. In the beginning of the book, the detective has multiple angles to pursue, only to find that all those angles are somehow connected to the killer, if only our hero is savvy enough to discover how.
Flow is one of the hardest aspects for me because I have trouble detecting tangents while editing my own work. This is my focus for the near future, and it’s why cutting is usually best when rewriting. Formula = first draft – 10%, according to Stephen King, and I find that 10% usually contains side adventures that I think are fun, but that have to go to make room for the actual story.