When you tell people about it, especially in detail, you get a sense of accomplishment that should only be yours when you've completed the task. 

So keep it under wraps and tell people when it's done so you can enjoy actually having achieved something and not just the sense of being so noble/motivated for having goals.

Edit: For people saying the opposite works for them, great! I'm glad to see you've found something that works for you. However, since this works for me, I'm going to assume it does for Some other people as well. Thanks for the comments though!