on Fiction University:

Many writers hate middles, and for good reason. But writing a strong middle is easier than you think.
When I was new to writing, every novel I wrote bogged down in the middle. I’d start off well, but then I’d run out of things for my protagonist to do, and I’d start making things up just to fill up space, and it would turn into a complete mess and I’d start over.
I can’t tell you how many times I did this. Dozens at least, if not more.
Eventually, I got frustrated and decided if I wanted to break this cycle of despair, I’d have to find a way to get through the middle of my novel.
And I did.
So well, in fact, that my agent said my first novel’s middle was “damn near perfect.”
One big reason middles are so problematic, is because the exciting beginning is over, but you’re not to the exciting ending yet. But this is where the meat if the novel unfolds, so slacking off just isn’t an option. What you need are a few bones to stick that meat to.