Many, if not all, writers -- published or not -- are looking forward to the day they can cut the chains of their "day job" and write full time. The cold reality is, probably never.
Of all the writers I know, only one is full time -- and that's stretching it a bit. Kevin (no relation) is a newspaper man; his day job is writing nonfiction, his writing writing is science fiction. Keith, one of the most prolific writers I know, would not make ends meet without his editing gigs. Scratch any writer and you'll find (just running through a list of my writerly friends) a psychiatrist, a software designer, a college instructor, two teachers, a graphic designer, a retail store manager, a retail store clerk, an IT guy for a large corporation, and a salesman. I'm a case manager for a family preservation services agency.
The reality of writing is that the income is not steady enough. Loan agencies can not include money from writing when figuring your debt-to-income ratio because past sales are not an indicator of future income. Which may annoy a writer fresh off his first novel sale, but is wise. In 2009 my earnings as a writer were about 1/3 of what they were in 2007; which is to say more than any year up to 2003, about the same as 2004, but less than 2005, 2006, or 2008. There's no pattern to my earnings, and without my day job there would be months with no income at all.
In her invaluable Freelancer's Survival Guide award-winning author and editor Kristine Kathryn Rusch does not recommend going completely solo until you have the resources to cover living expenses, medical insurance, potential emergencies, et al., for at least a year set aside. By that standard, I see no way I will ever be in a position to write full time, unencumbered by a day job.
Does that mean don't even try? Of course not. The year of my first sale, 1999, I was in mental health, working with at-risk youth from 8AM-4PM five days a week, had a paper route from 3-6AM seven days a week, and taught English as a second language at the community college from 6-9PM three nights a week. I wrote on weekends and during the evenings I was not teaching. I mark the beginning of my professional writing career from 2005. That was the first year my income from writing was such I could afford to give up the paper route and the teaching.
A few rules, a work ethic, enabled me to reach that point.
The first rule, of course, is to write. I used a label maker to put a strip across the bottom edge of my laptop's screen: "Write. Do not read about writing. Do not chat about writing. Do not post about writing. Write."
The second rule is to treat writing like a job you need. As a paperboy (excuse me, media distribution specialist) I had to get up at the crack of you've-got-to-be-kidding-me and despite cold or storm or not feeling well, get 300+ news papers rolled, bagged and on the right porches before sane people were out of bed. There were no excuses for running late and calling in sick required a doctor's documentation in 12 hours or termination. Applying those standards to your writing schedule (i.e.: If you have a pulse, you are typing) can do more for your writing career than any other single factor.
The third rule is to have a plan. Set realistic objectives and have a clear-eyed understanding of the steps you are going to need to take to reach them. Here I was greatly helped by my wife Valerie's budgeting skills and the fact she will tell me what's what without hesitation or blink. If I had followed my instincts -- quit the paper route after my first few sales -- I would have been back in the part-time-job market, using up my writing time trying to find work, in weeks. (And stressed about bills we could not pay.)
I currently have a plan in place with a two to four year timeline (that flexibility reflects several factors over which I have little or no control). Somewhere (somewhen?) around 2014 you will find me teaching classes on creative writing (hopefully at a university, but a community college is fine) and spending the bulk of my time on writing original fiction.
I'll let you know how that goes.
In the meantime:
Treat writing as a job you need to have.
Set realistic objectives, and develop a practical plan for attaining them.
Take each step one at a time, and you will become a professional writer.