Introduce the Unknown and you will have the reader on edge — an immediate increase in
reader tension. You can do this at any point. I think Middles are particularly well-suited for doing this since Beginings are for set-up, Endings are for wrap-ups, but Middles are for complications. And what is more intriguing and unsettling to the reader than an “I know something you don’t know”? Especially right when the reader is getting complacent.
Foreshadowing in prose, allusions in dialogue, or unexplained events are just a few ways to raise reader tension. Some of this might be plotted into your first draft but if you have places that sag but are necesary to the story, you can add elements of the unknown to draw the reader back into the story.
Look for ways you can rewrite the scene so the characters explore a new area (a cave, empty building, new sector of the city. A place neither the characteer nor the reader has ever been before.
Look for places where you tell back-story or history directly and without conflict. Or brainstorm back-story or history that is not included in the story but you wish you could have included. Turn this into a secret to be kept from the reader (and maybe the character) that must be discovered through hints and clues given out by various characters and settings.
Take a character who is important (but maybe not vital) to the story and rewrite your manuscript so the reader never actually meets this character.
Take your time when introducing a new character or setting. The reader doesn’t need to know everything at once. In fact, the reader doesn’t need to know everything at all just so long as you know. Show the action and take your time with the explanations. (Some explanations can wait longer than others.)
Introduce uncertainty, mystery, indecision — the unknown — to increase reader tension. Raise reader tension to help tighten the sagging, dragging passages of your manuscript.