In my opinion, the toughest thing to write is a short summary of your novel. As a general rule, this summary might be about two hundred and fifty words long, and it's the copy you might see on a book jacket. It's your book in a nutshell.
When I wrote my first book, I made a mistake. I wrote the summary after the book was finished. Never again. With the main plot, the subplots, the twists, the inner conflicts, the external conflicts... it was impossible to know how much to say and what to leave out. I promised myself that if I ever wrote another book I'd write the summary first.
But I am such a liar!
Well, sort of. I spent an entire flight from Houston to Washington, D.C. writing a two paragraph summary for my second book. I liked how it turned out. But later I re-started the same story with different characters (see I have a date with my WIP) and I never fixed that summary. Now it's all wrong for the new story.
I aim to fix this before the current book gets to the halfway point. After all, having a summary in hand might help keep the story on track. I've been working on it today and, even without a finished manuscript, these are still the hardest two-fifty. Incredible.