Some great posts exist on middles, but this post from Lisa Gail Green was written just a few weeks ago. You should read it. It's short (her posts usually are--I like that!), but it has great depth. She lists three simple questions to ask ourselves about our middles. Those three questions focus on: moving the plot forward, building character arc, and creating tension. If our middle does those things, it will not sag!
But I've read books, and you have too, where the middle does sag. The problem I find in my own writing is that a saggy middle is SO HARD to recognize. I know the characters, I know the outcome, and there's this super cool path the characters must tread in order to reach the outcome. And I can't possibly cut anything because, for sure, none of it sags, right?
Yeah, probably not true. So, how do you recognize sagging portions of your own writing?