When I sit down to write, I am usually in one of three states: unrestricted flow, restricted flow or no flow.
Unrestricted flow is the holy grail of writing when the words come easily and without restriction. It is the condition when the words feel like they are coming from another place, like you are merely the vessel through which they flow. (I don't believe it's anything quite so mystical - they are just coming fresh from the unconscious mind.) This happens to me a lot - in fact, I wrote an entire novel in this state. By analogy, I do jumbled word puzzles, and can often see the answer without consciously thinking about it a fraction of a second after I have glanced at the group of letters.
Restricted flow occurs when the ideas stutter. They still come, but not as quickly, and they require more editing. In this state, I am also more conscious of craft: I think a lot more about which type(s) of humour would work best in the given situation. I also sometimes have to leave holes that I will get back to. In the jumble puzzle analogy, I have to play with likely letter combinations, sometimes coming back to the letter group several times before I finally see the solution.
Finally, there is no flow, which is as dire as it sounds. Nothing comes. Some people refer to this as writer's block. Fortunately, it doesn't happen to me that often, and usually only for short periods of time. Unlike some writers, I walk away from the keyboard rather than frustrate myself trying to come up with something when there is nothing there; my time is more productively spent reading or doing background/business writing (ie: catching up on email or social networking) when I'm in this state. Sometimes, the answer to the jumble puzzle just doesn't come.
I have noticed that these conditions are related to my physical condition, especially how much (or, how little) sleep I had the night before. This is probably my biggest motivator in getting a good night's sleep!