A few weeks ago, we talked about beginnings—the best place to start, and how to draw your reader into the story. You may or may not find beginnings easy as a writer. But most writers would agree that middles are more of a challenge.
This is where the story starts to drag. It may also be the place where your characters start to take you in a different direction than you intended. So what do you do?
First, talk to your characters. Interview them, asking them difficult questions. Don't ask, "How was your day?" Rather, ask them "What was the best (or worst) thing that happened to you today?" Their answers will help guide you.
Next, write down the beginning and ending points of your story. Your characters started at Point "A". They need to get to Point "C". What are all the possible Point "B"s that will get your characters to the end you want for them?
For example, at Point "A", your protagonist finds himself on the edge of bankruptcy in the family business that's been around for over a century. Point "C" is finding the funds to save the business. Point "B" may be winning the lotto (highly unlikely and too contrived), winning the lotto, then losing the money in Vegas (back to Point A), or asking your arch-enemy's daughter to approve a risky loan.
There are any number of possibilities. It's up to you and your characters to choose the right path.