Once the introductory scenes have been completed. This could take a chapter or two of the book since you're bringing in a lot of information in this area. The characters, a setting or more, the characters' initial wants and the internal and external complications have all been touched upon. Then comes the middle of the book. Now this part of the book can sag but preventing that is thinking about the middle of the book as a time of discovery.
In a romance, the hero and heroine are learning things about each other and how their own wishes and desires effect each other. They are also having to look at the external conflicts. In one of my stories, the hero and heroine are very attracted but they're on opposite sides. She's union and he's management and this can be problems. During this time while their romance slowly develops, they also isolate the external problem and find they're both on the right side. Now they must work together.
In a mystery, once the crime has become evident the characters must work through the initial investigations to find the real root of the problem. Here there are usually one of more characters in direct opposition. The detective and the criminal. So the middle scenes will pit the against each other while they learn just what is at stake.
The middle pattern of discovery follows through all kinds of genres. The middle can's sag if you remember that the characters are learning more about each other and about the vaious conflicts both internal and external.