Are you calm, relaxed, and a little bored while writing? Or hyped up and nervous? For me writing is a very intense activity. I wouldn't want to do it more than 3 hours a day, and two are even better. There should be a little adrenaline flowing, but the ideal should be a flow of alert but still relaxed attentiveness. It might be hard to cultivate this, because most of the time you are writing you probably won't feel this way. You still have to plug on through various moods rather than waiting until you feel this way (you might never).
What I recommend is that when you do hit that sweet flow, notice it and remember it. What made it happen? Maybe it was something that kicked in about 45 minutes in, like a runner's endorphins. How long did it last? 10 minutes? An hour? Once you've figured out onset and duration, you are in a position to exploit those states when they do come and maybe recreating those conditions. They are doubly good: they can produce good work, and also be motivating since they simply feel so good.
It is a mistake, though, to think that good writing can only flow from maximal flow states. In fact, I think i am usually not in one, yet still can produce good results. I think I am too often just hyped on caffeine and way too anxious. The trick is to have a balance between cultivating good writing moods and not letting their absence get in the way--the latter phenomenon is one of the main causes of writer's block. If you wait to feel good to write, you are making a mistake, because the good feeling will only come once you've been writing for a while.