Sometimes something comes to mind, sometimes it doesn't. Tolkien took over 10 years to figure out the LOTR world. I bet that's not because it all just came to him at a peaceful moment and all he had to do was write it down. It's one of those things everybody knows is myth, but somehow still tries to live up to.
I get the best ideas when I don't have time or the chance to write them down at all! When I have time and feel good and relaxed and am in mood to write, nothing comes to me. Pretty much annoying, and probably the reason why I could never do something creative (in my case writing) for a living :)