I love this blog post http://blog.asmartbear.com/your-idea-sucks-now-go-do-it-anyway.html
"My idea isn't good enough yet" explained a friend who is thinking of starting his own company. He was waiting for the idea to be completely fleshed our before taking the leap.
Here's a newsflash: Your idea probably sucks, and it doesn't matter because your business will probably turn out to be something completely different.
Check out the examples in the post about how Paypal and Flickr got started, wow, who's kicking themselves they missed out on those opportunities because the ideas were still embryonic.
Guy Kawasaki has a similar principle in his "Art Of The Start" which is basically get started, your first few attempts you are going to screw them up anyway and the sooner you start the sooner you can get them out of the way.
Great words to live by.