(This is an abridged version of a plotting handout.)
A common problem for many writers is the "sagging" middle. You have a great set-up and a great ending, but getting from A to Z turns into a muddy morass. The pace slows, the characters seem to be wandering around doing a lot of navel gazing or rehashing issues... What's an author to do?
* Add complications and conflict. Don't settle for one internal conflict, or one issue. Ditto the external conflict. Let your characters solve some problems, then add new ones.
* Make your characters active --
There are two kinds of action your characters can engage in: physical and mental. Physical action is, of course, things your characters do with their bodies, such as running, walking, eating, making love.
Mental action is just as important, and the most important mental action your characters can take is to make decisions.
decision = action = reaction = decision
With every decision, the stakes should get higher, the decisions more difficult.
* Make sure things don't happen just TO your characters, but BECAUSE OF them.
* Reveal motivation and back story gradually, at different times and places. This is why you don't put it all in the beginning -- these revelations can keep your story going in the middle.
* Your characters should be reaching new levels of intimacy, physical and emotional.
* The consequences of the conflicts become more serious, whether physical (risk of bodily harm) or emotional (a broken heart).
* Be careful with flashbacks, because they disrupt the flow of the narrative and can really mess with your pacing. Note I'm not saying never use them. Sometimes they're great and necessary. Just be aware that they can cause pacing issues.