Applelight: To add more to the "just do something" pile, I recall a bit of advice from the blog of an amateur SF writer. He said (as best I can remember it) "The difference between writing your story and talking about your story is the difference between having a long-term romantic relationship and having a hand." In other words, talking about ideas gives you a quick high—not really as good as the feeling of properly birthing a finished story, but a hell of a lot easier to achieve. And that easiness makes it tempting to keep talking and never get around to writing.
In my own fanfiction career, I've taken that advice to heart and I only talk about unwritten portions of my stories in the vaguest of terms. For
Alarm Clock, the first thing I ever said to anybody about it was, "Has someone already done a fanfic about Derpy being able to see magic and other dimensions?" The very next thing I said about my story was, "I've written two chapters; can I get some feedback?"
Furthermore, I'd like to second what Jteeth said about pinning an idea down and just writing it. You've heard about
Duke Nukem Forever, right? Do you know the reason why it languished in
Development Hell for 15 bucking years? Basically, the project head wouldn't settle on a specific vision for the game, just the idea that it would be the greatest FPS ever. So he was constantly getting new ideas to incorporate, long after the point where development
should have been too far to make changes. Which led to a vicious cycle where other games would come out during
DNF's overplanning-induced delays, then the project head would see those games and get
more ideas to add, so
DNF would get delayed more, and then other games would come out...