Entersection has an excellent post on establishing habits today. The last sentence particularly resonates with me:
"Every time a resolve or a fine glow of feeling evaporates without bearing practical fruit is worse than a chance lost; it works so as positively to hinder future resolutions and emotions from taking the normal path of discharge."
I've experienced this phenomenon of lost momentum many times in my writing. When I am working daily on a project, I am more likely to continue to work daily on it, to flesh it out, to be excited by it. But sometimes I get off track--I have too many other tasks that day, and I end the day too exhausted to produce writing. At the time, I often think, "Well, it's only one day off; I'll just pick it up tomorrow". Then the next day comes, and I have a harder time lifting up the end of my work where I left it, finding that mental groove in which to work.
That's why habit is essential. For me, the hardest part of finishing a project is the physical, mundane action of sitting down to write the thing. The more often I am able to finish a successful writing session, the more likely I am able to pick it up again the next day. Past success bolsters me on to future success.
On that note, I should get to work.