But what about the middle? Personally, I've always felt that I'm all right at writing beginnings and ends. What I really struggle with are middles. How do you keep up the tension in a story while continuing to develop character relationships? How do you introduce plot twists without it seeming forced?
Take a book like Lord of the Rings. As epic fantasy, it follows a plot line that consists of multiple small incidents leading up to a bigger one. In the Fellowship of the Ring, Frodo and Sam must escape the black riders, make it through the old barrows, survive a trek through the wild with Strider, traverse the dwarven mines, and thwart the Uruk Hai sent by Saruman. Each of these incidents function separately from the others, like a series of trials. This goes back to the work of Campbell and the idea of a hero. Frodo's journey follows a "quest" structure typical of Tolkien's genre.
Harry Potter, on the other hand, employs different tactics to make the middle of the books interesting. Instead of smaller, separate trials, the Harry Potter books generally focus on a single overall mystery, and the characters uncover pieces and clues as the books go along. The books also employ subplots surrounding the characters' romantic lives and struggles in school. It is more character-driven than Lord of the Rings; although the fantasy aspect draws readers in, so too do the typical teenage problems of Harry, Ron, and Hermione. It is, in a way, a more complex method of increasing tension than the methods employed in Lord of the Rings. Of all the Harry Potter books, Goblet of Fire follows a structure most similar to that of epic fantasy.
I think the key to making the middle of your book interesting is to never let the tension dissolve. No matter what's going on, you need to have some kind of suspense, be it romantic, physical danger, school-related, or a mystery. There are large sections of Harry Potter in which nothing Voldemort-related happens. Voldemort is an obvious source of tension, so JK Rowling compromises with different problems. Tension doesn't have to be all dragons and wars; there are more subtle forms that will still hold your readers' attention. Epic fight scenes and immediate danger are easy ways to build suspense, but generally speaking, too much danger in a book will feel forced and fake. In writing a good middle, the key is discovering how to build up subtle tension whenever there's a lull in the action. Identify your characters' inner struggles. Use their emotions to drive suspense in the story, and you shouldn't have any problem hooking your readers from start to finish.
What about you guys? Do you struggle with beginnings, middles, or ends? How do you deal with these issues?