A reader wrote me an email saying that really wants to write a book and has started several--but he never finishes any, mostly because as soon as he starts, he has another idea that seems better. Here's the advice I gave him--maybe it will help you as well:
Make a list of the ideas you are considering. Pick the three that you think are the best.
A week later, look at the three (and only the three-not the ones you didn't choose). Pick the two that you think are the best.
If you have anybody you think has good judgment about the type of book you are thinking of doing, ask them which of the two they would find most interesting. Giving them a choice like this means they have to think about it, they can't just say, "Yes, I like that idea." If they say they like both, say if they had to choose one, which would it be? If you can do this with three or four people, so much the better. You don't need to tell them the whole idea, just summarize it in a few lines.
Taking your own intuition into account, as well as the opinions of these people, pick the one you are going to do. Make a commitment to yourself and maybe to others that you are going to finish this project.
Then write that book. When new ideas come up--and they will---just jot them down and put them in a box or folder. Even if one of them seems obviously better, stick to the one you started to do.
As you continue to work on that book remind yourself this is a learning experience. You will learn a lot more from finishing one project than from only starting three.
I don't know whether you've read my newest book, "Your Writing Coach," but you might find it helpful--it talks in more detail about motivation, persistence, etc.
Good luck and send me an email when you've finished that book so I can congratulate you!
(Get more creativity and productivity tips from my free monthly Brainstorm e-bulletin--all you have to do is request it by emailing BstormUK@aol.com)