Narrowing Your Story

Often you might run across a writer who doesn't want to share his or her book idea. Afraid to share the information, the writer considers that someone might steal the idea or is superstitious that by talking about the idea, it will never come to fruition.
Currently, I'm working on a book proposal. It is an idea I've been focused on for a while and involves a high school friend I reconnected with a couple years ago. My friend doesn't know much about publishing or book proposals, so I'm putting it together. As we've bantered about the book, the concept has fleshed out. I'm confident we have a book.
During the important stage of putting the book idea to paper, I started exchanging e-mails with a friend with dozens of successful book proposals under her belt. She is graciously helping me put together the proposal. (I've done proposals before, but they have yet to risen to the level of her successes.)
I'm so glad I consulted with her.
Since we spent so much time considering the book, we had accumulated too much idea. The book had become unwieldy. In fact, according to my friend, we had two books on our hands.
Try to succinctly explain your book idea to someone really helps solidify your idea. The laser focus you can bring while giving someone the bare bones of your proposal can help form the idea, while also giving new energy to the project.
My friend was able to help me tease out the important bits of the project and left me with an enthusiasm to bring the proposal to fruition.
Do you have anyone you can turn to and explain a book idea? If so, I highly recommend starting the conversation.
If you are concerned about your idea being stolen, write it down on a piece of paper, date it and mail it to yourself. Keep the envelope sealed. Just in case you need to prove when you came up with the idea, the postmark serves as your proof system.
As for the superstitions, I'm not sure how to hold them at bay. Any ideas?