Write about what you want to read. Write about what you want to see in a book and write with character's you would want to read about.
Have a solid outline. It helps to see the process of the book you are writing when you are writing it.
Do a lot of brainstorming and do not be afraid to go in the direction your story is pushing you in. Sometimes the ideas that seem that they will not work at all are the best ones.
Don't be afraid to experiment with ideas. Try out new plot points and introduce new characters.
Develop your characters. Ask yourself questions about them to picture their motivations for their actions in the story.