In the end the most important thing for a professional writer isn't his skill, or his subject matter, or even his appeal to the market. Poorly written books sell if marketed well, any subject matter can be appealing to someone, and even if you don't have much appeal, sheer persistence and amount written can make you those precious dollars in limited markets.
The most important thing the professional writer needs is good writing habits. This is true for two primary reasons:
1.) Without good methodology, you never get anything out in the first place.
2.) Without good methodology, you'll end up with either gaps in your writing schedule, or frantically trying to find time for everything you want to write.
You may have heard the old excuse "That project is on the back-burner for now." More often than not, what this actually means is they haven't even thought about it for month. Whether it's a story they're "writing," fixing that car, or cleaning out the garage, it's out of sight and out of mind.
Well, as a writer with a good methodology, I always have something on the back burner, and that's a-ok.
Let's approach the metaphor: The standard stove top has two burners at the front, and two in the back. Huh. What do you know. One of those front burners is usually a little bigger than the other burners. They have different uses.
The largest burner is for your main dishes, usually. This is your stew pot, or that giant frying pan. This is where most of your attention is. The one to its right is often used to boil noodles or make rice or something else dreadfully important to your meal.
But what of the ones in the back? They're usually at low-heat, either keeping something warm or slowly cooking something that isn't meant to be done real fast [i.e. a sauce, or gravy] or something that doesn't require very much attention from you [canned vegetables, anyone?]
I approach writing from a similar angle: the stove-top is the work-space for the entire meal - my career as a writer. The big burner has my current major project on it - for a lot of writers this will be a novel, for others this is that big thesis, or an anthology. It's the main course for this phase of your professional life.
To the right, always in my mind, are my noodles and rice. Important things that are oft overlooked - short stories. When-ever I can break away from my novel, I work on short stories for submission. These are important to any writer who doesn't have just the most awesome luck. These get the word out: I'm here!
Unfortunately, only to the kind of person who reads short stories. Usually other writers, and people in the publishing community who subscribe to or edit the magazines that buy them. Similarly, only a gourmet is going to care what shape those noodles were or whether the rice you used was short or long-grained.
But what of the back-burner?
These are projects on low-heat, they don't require my immediate attention, but they're never far out of mind. See, there comes a time when you're going at that main dish, stirring it and mixing in spices and - huh, you've just kinda gotta let it sit for a while. You're just being a pest.
This is when you check the back-burner. Your gravy from scratch is slowly bubbling and solidifying from that nasty mass of stock and bits into something edible. Maybe it's time to add some fresh mushrooms. The veggies you were steaming back there are pretty tender, maybe its time to take them off and turn them into a finished product.
The metaphor kind of breaks down here, actually, because the food parallel has nothing to do with what kind of project is actually back there. For me, I see the gravy as a novella I've had going, and the veggies as a project in editing phase, but the fact is you can have any type of project on that back-burner. What's important is that you have something there, and are actively working on it in one way or another, but with your attention weighted towards your current highest priority.
For the tl;dr crowd, what this means is that I personally always have four projects going. They don't interfere with each-other in the least, and when well managed they complement each other like any good meal.
Another word for this behavior is "rotation," actively filling in the empty spaces where you can't be working on one thing by working on something else. It ensures that there's no gaps that you didn't personally schedule [everyone needs a vacation sometimes.] Also, having a limit on those side projects, both maximum and minimum keeps you from being overwhelmed by all those other wants.
So, my fellow cooks, put on your chef hats and aprons, and keep cooking with all the resources you've got. Never turn off those burners, and keep 'em full.