I’ll second (or third or fourth) writing every day. I don’t write a whole lot of fiction - I usually sell business, travel and health magazine articles plus business copy for Web sites, marketing collateral, retail copy, etc. (copywriting is definitely my bread and butter, but my more creative stuff is becoming more successful), and I’ve found that writing every day is important. Getting into a schedule often helps, simply because it becomes a habit to write, and the more experience you have writing, the better the writer you become.
I’d also recommend reading a lot, both of what you write, and of everything else. Not only will you find out what sells by reading what you’d like to write, reading everything else can give you new ideas and spins, and can also make you a more flexible writer.
I would also second Euthanasiast’s recommendation that you get a copy of The Elements of Style. And when you’re ready to sell, get a Writer’s Market to find places to buy your work. Plus, if you have problems disciplining yourself to write often, take a class at a community college, or even one online, so you have to write.
It can be tough making writing or editing a profession. You’ll get a lot more rejections than you will get acceptances, so it’s important that you develop a thick skin. It’s also important that you research your markets carefully and find out what a particular publisher is looking for. If you ignore their guidelines, your submission or query will go straight to the trash.
So, write what you love, write about it often, read other writers’ work, and pay attention to what publishers are looking for, and before you send it to them, edit your work multiple times, or, if possible, run your work by a friend or ask a professional proofer to look at it for you if you’ve got the cash.
I’ve only been writing professionally (meaning, I’ve got my own writing business now and am beginning to make money off it) for a year and a half. The first year was very, very slow, but after making an effort to network, and many carefully crafted query letters and sales letters later, things are picking up and my business is beginning to take off. I wouldn’t give up your day job just yet unless you’ve got enough money stashed away for a year or so, but you can make writing or editing a full-time profession. Just don’t give up and keep writing.