Don’t allow your reader to skip the middle of the story. Throw plenty of action, new characters, new developments, new settings, twists, and turns at the reader.
The middle should have a crisis. It could be physical, emotional, or both. Think of ways to surprise the reader. Intensify the problem. In a journey story, supplies run out, or someone runs off.
Does the main character have a set back or doubt his abilities and what motivates him to move forward in spite of his doubts? Show another side to his personality.
Take the reader inside the hero’s head to show fears, misgivings, and secret vanities. Give a character a secret, build a lovable quirk, or create an unexpected character with an unpredictable role.
Does the antagonist have a presence? Does he have any phobias, weaknesses, or shortcomings to overcome?
What is the exact opposite that could happen at this turning point? What is the most outrageous thing that could happen at this point? Think of a cliffhanger and its resolution.
How does the conflict affect the characters and plot in both the short- and long-term of the story?
Add a momentary triumph followed by a setback. Think of a subplot that adds to the story.
Alternate the action or description with building the character’s history and relationships through inner thoughts. Adjust the pace of the story with dialogue to move faster or introspective to slow down.
Stretch the tension out in the middle and make objectives clear to the reader. Emphasize the importance of achieving the goal.