There's somethng I do before starting to write a book that usually keeps me focused on the story. Note, I say sometimes. There are times I've been diverted from the story and that's usually because as what I've set out to do has evolved into more than one story.
When I started writing, I came across a book by Dwight Swain that was a great help while I struggled with learning to form my ideas into stories. I wish I had the book but I loaned it to someone and haven't received it back so I'll try to explain in my less that wonderful way and issue a challenge to fellow writers.
What one does when the ideas and characters are bubbling in the thoughts is to write what your story will be in two sentences. One is a statement and the other is a question. This is sort of a synopsis in two sentences. Here's the one I wrote for one of my latest releases, The Warrior of Bast. This was done before I began the story.
Statement - On the run from she sister's killers, Tira reads a flyer offering an escape and a chance for her to fulfill a dream.
Question - As a warrior of Bast with the help of Kashe, a warrior of Horu, can she find the missing symbols of an ancient Egypt and defeat the priests of Aken Re?
The challenge is this during the next month, write your own two sentence synopsis of the story you're working on or one you plan to write and post it on your own blog or put it in the comment section of mine. For blog followers, there may be a surprise.