Here's how I do it. Your mileage may vary:
- Treat writing like a job (because that's what it is). I write from about 9:00 (after I drop my son off in kindergarten) to around 4:00 or 5:00. I'm no longer working evenings and weekends (well, most of the time I'm not -- there are days when I really want to finish something and I need to work longer, but that's the exception rather than the rule). If you just sit around and wait for inspiration to strike before you start writing, you will probably never finish anything.
- Take breaks, and get out of the house! I schedule lunches with friends two or three days a week so I can get out of my office and have interaction with real people. (I like solitude, but not that much!)
- Have a goal or quota, and don't stop until you hit it. Before I started writing full-time, I set a goal of wanting to write at least 1,500 words per day (you might use number of pages or some other measurement, but the idea is to pick something). That would keep me on track to finishing a long novel in about a year. So far I've been hitting 1,600 to 2,300 words every day. As I've said before, outlining really helps!
- I like listening to music while I write. Others might need absolute silence. I've been burning my CDs down to iTunes while I work (2,000+ songs so far) and I just hit shuffle and let them play. Or I listen to some of the radio stations on iTunes. There's a pretty interesting selection of stations out there.