A writer asked me today if I had any advice on how to summarize a story into 150 words. This is what I told her:
Take a book you love and know well, really well. Write 150 words on what that book is about. You'll be able to do it because you love and know the book.
Now apply that same skill to your book.
It's been said by many before me, but I say it here again: If you (or anyone) are having trouble summing up your book, is it because it lacks focus? Just a thought.
Write those 150 words as you would tell it to someone who asks, What's your story about?