I have to be honest. I've been busting my brain all day trying to come up with something that Jody and Talia haven't already said about the sagging middle. My blog-sisters did such a wonderful job, I'm left with with giving advice. For what it's worth :)
I never got the whole 'sagging middle' thing. After all, if you have a good plot and solid interior and exterior conflicts you can avoid it, right?
Sure. But when I see a middle sagging it's usually because the writer is adding too much to the plot. Things that can be easily left out without endangering the quality of the story.
Only once did I have trouble with the middle of a book. I agonized over it for years. Until one day I decided to just skip the middle and write the end. I can always go back and fill in later (something I loathe to do)
What I found was I didn't need any more than what I already had. I thought all my books had to be at least 80k words, but in reality, some stories just aren't that long.
There are plenty of books I could name that could've done with a bit of editing, subplots that could have been omitted. In the end it's up to the author and his/her editor to decide what is needed and what must go.
If you find yourself rushing through a scene or skipping it all together, get rid of it. Never give them a reason to put the book down. Always keep them coming back for more and you'll never have to worry about the sagging middle.