Member Rara Avis
At the risk of mixing our metaphors, I think your window is opened by a mental muscle - and like all muscles, grows stronger with use. Unfortunately, to extend my metaphor, there isn't a muscle in the human body that doesn't have a countering muscle, the pull to every push, and I guess mental muscles are the same. For me, I find the muscle that opens the window often has to fight against the muscle seeking perfection. If I put too much thought into how "good" something is going to be before I write, well darn, I never seem to get it written.
Now, if you'll excuse me, I need to go find some Ben-Gay.