Over my short career as a writer, I have been very fortunate to hear editors say 'Yay, no saggy middle issue', but believe me it's not because I don't feel the pain of that middle section of every book I write...I've just learned to 'get around it'.
Despite seeming to not have this issue with my writing, every time I attend a conference, I purposely sit in on sessions that deal with this because as I said-I find middles just as tough to write as any other author. And I always learn something new. So, I thought as a lot of us are writing new books right now, I'd share some of the techniques that I've found most helpful.
-Raising stakes-We all do this at the beginning of the book, but sometimes an unexpected twist in the middle that makes the protagonist's goal that much more important can help you get through the middle. Things like a compressed timeline-a deadline that is now even shorter or involving a loved one-someone else who is affected by his or her actions can add a richer element to the plot and keep the story moving forward.
-Throw a Curve ball-Have your protagonist realize that the goal they've been chasing is the wrong one and have them switch focus. Alternately, you can make the goal harder to attain or remove it entirely-now the character has to go in a different direction.
-Cripple your Hero/Heroine-In every story, the protagonist usually has things they can depend on...things helping them achieve their goal. Take these away. Remove allies-maybe the friend betrays them or leaves them. An ability they rely on could also be taken away. Ex. They are a runner-they break a leg.
-Switch characters- If a particular scene in the middle isn't working-switch out a supporting character. Choose the least likely character that you would have normally put in the scene and watch it take the plot in a direction you'd never have thought of.
-Skip over it-I know we've all been told that writing out of order is a bad thing-but it works just fine for me. Often, I know what scenes NEED to happen in the book, the ones that really drive the plot, the ones that excite me-so I write them first. I know the ending always as I begin a book, so usually I write it before the middle and then work backwards. For example-in book two of my series-I needed the heroine to be in her garage (she's a mechanic) toward the last scene of the book in order to make the ending work, so I wrote that scene, then figured out what scenes needed to happen before it to get her there:) Ex-she forgot something...so I needed to figure out what she'd forgotten and how that item got there in the first place...and so on...
I hope this helps:) Happy Writing!!