BLUF (Bottom Line Up Front): Write more by stopping when you want to keep going.
I quickly answered a question from a member of the Horrible Writing Writers Support Group on Facebook last night because I was out having a beer to celebrate the fact that I sent my next novel off to my editor. I didn't do it justice because it's an answer I feel needs more attention because it can be incredibly helpful to all (but especially to new writers).
In terms of keeping momentum going, slogging through those middles or when you just feel like you can't figure out how to pick up where you left off, Hemmingway had a trick I incorporated into my own writing and, I'm telling you, it increased my output 10-fold.
What is the trick?
During your writing time, end your session at an exciting part. It could be an action scene, a major plot twist, a dramatic comment by a character, whatever. You could literally end your day's writing at, "Jimmy didn't see it coming, but when it slammed into him with the force of Thor's hammer he ....." You'll be thinking about what Jimmy did ALL. FREAKING. DAY. LONG.
The key is to stop writing at a point where your pulse is surging, where your mouth is dry with excitement, where your fingers can't keep up with your brain.
Sure, you'll be tempted to keep writing, because you're chasing the high. And that's the genius in Hemingway's tactic.
When you stop at the point where you NEED to keep going, it will percolate all day/night. It'll be the last thing you're thinking about with your head on the pillow and the first thing you think about when your eyes snap open in the morning. You won't be able to get back to the keyboard quickly enough and you'll find, when you do finally lay fingers on it again, that you're off to the races once more.
Your productivity will increase because you're no long slugging through all chapters or the past day's writing to remember where you were.
You sit. You write.
I dare you. Try doing this for 30 days and come back and tell me what you noticed. Be faithful. You MUST stop each day at an exciting part. Try it and see how this changes your writing life.