Two weeks ago I talked about plot being two dogs and one bone. What that means is plot is essentially conflict and conflict causes tension. It keeps the reader turning pages because they want to know what happens next. Afterall we all slow down at the scene of an accident because we want to know what happened or if we can help. Whether you are a plotter/outliner or a pantser (free style writer), you need to keep an eye toward tension and conflict in each scene.
There are many different kinds of conflict and tension. First there is the story conflict--you have this in every book of every genre. You have no story without an over arching conflict whether that is looking for a happy-ever-after, solving a murder, surviving a bad guy, winning an epic space battle, or making it out of the 5th grade. Each work of fiction begins with an over all story conflict. Make sure you have one. It's not enough to simply have a cool setting or funny characters. The reader wants to know what happens and why they care.
Next there is relationship conflict/tension. I'm not talking about only romance. Relationship tension works in all genres. Think Harry Potter and Professor Snape, Luke Skywalker and Darth Vader. A story really hums when you set up opposites, and pit them against each other, give them the same goal. If you find that you have a sagging or dull middle to your book, look at your character relationships and make things worse, ie. the bad guy is winning, your character just crossed a personal line they thought they'd never cross, someone they love gets hit by a bus, or thrown in jail and the character can't fully participate in helping. Tie them up in emotional knots or real knots and make them watch while others hurt/fall/struggle.
No where are we watching characters in day to day activities...getting dressed, making dinner, paying bills... unless it adds to emotional or physical conflict and tension. They have nothing appropriate to wear, their boss is coming to dinner and it burns or there is not enough money for all of the bills.
I recently read Jeffery Deaver's Roadside Crosses. Here are some examples of tension in this book: 1) overall story- someone is hurting people escalating to killing people and leaving a roadside cross announcing they are about to do so. (The roadside cross in advance of the crime gives the feeling of a ticking clock which hurries the plot along. The escalation of violence from kidnapping to torture to murder also speeds the plot-things are getting worse.) 2)conflict between heroine and her part time partner is two fold- they work well together, like and respect each other, there is sexual tension...and yet he is married. Plus he has his own cases to work. 3) heroine's mother is arrested for murder unrelated to the case and heroine can't devote all her time to helping her mother because more people are getting killed creating guilt and another strained relationship. 4) heroine's boss comes down on her for political heat he is getting on the case. 5) Another man enters the picture who helps with the case and shows romantic interest-but will her children approve? Is she willing to let go of her feelings for the married guy? 6) There are protesters-zealots ready to picket and mob anyone that opposes their views.
These are great examples of story question/conflicts/tensions large and small that create pacing, and keeps the reader turning the pages. These characters are feeling conflict at every turn. They are having a string of bad days...bad days that eventually work themselves out.
So, if you're stuck with a sagging middle or you don't know what happens next, think conflict, tension and problems great and small. Let the bad guy win. Let a deadline lapse. Let them fall for the wrong person. Let conflict carry you through.