I've been writing long enough, I think, for a worky-ass post about "process" that bores everybody to tears. So here goes.
There are three main reasons I don't write for long stretches - I don't like what I'm producing, I'm not getting ideas, or I am living too structured of a lifestyle.
The third issue is the toughest, and it tends to flow into the other two after a pretty short while. I work best in a strictly structured format when it comes to everything BUT writing, so if I want to run my life well, I need timetables and lists and charts and plans.
Writing is something completely different. My process is, shall we say, a disheveled process. It happens late at night while normal people are in bed, and it's characterized by a lot of pacing and absentmindedly leaving shit lying around.