All of the above assumes you're able to produce books that sell.
Here's what you do to keep yourself in business.
Write every day even before you quit. If you aren't
able to make yourself write regularly before you quit your day
job, the odds are that you aren't going to be able to make yourself
do it after you quit. Writing is a business for self-starters.
It's hard. If you don't know beforehand that you can sit down
and make yourself produce pages in spite of weariness, boredom,
lack of inspiration, or over-abundance of distractions, don't
kid yourself that writing full-time will magically cure this.
The weariness, boredom, lack of inspiration and plethora of distractions
will still be there once you quit, and along with them will be
the pressure of knowing that at least one person in the world
is now counting on you to put words on paper---and make them good
enough to sell---no matter what.
Do not let yourself forget that once writing is your job, it is
exactly that. A job. You have to sit down and do it when you don't
want to, when you do want to, when you feel crappy, when you feel
great, when the sun is shining outside and you can hear a mockingbird
in the tree and you know that the fish are biting down at that
shady spot in the river. You will, as a successful writer, work
harder for yourself than you ever did for anyone else. You'll
work longer hours. You won't be able to do a half-assed job on
the days when you really don't want to be there, because if you
do, the only person you'll be hurting is yourself. Yes, you get
to take days off whenever you want, but remember that they aren't
paid vacation days anymore. The person who pays for your days
off is you, so don't take too many.
Get used to giving yourself a page quota. Mine varies
from book to book and from deadline to deadline. It's been as
few as five pages per day and as many as twenty. I'm comfortable
at ten, I'm tired at fifteen, I thought I was going to die the
time I had to produce twenty finished pages every day. Five, which
is the page quota for my current book, is wonderful, and I find
myself going over it some days just because I can do it without
feeling strained.
Act like you're in business. Make a habit of meeting
deadlines. Get a reputation for being pleasant and easy to work
with. Take reasonable suggestions, and deal with suggestions that
don't work for you in a calm and reasonable way. For God's sake
don't become a pain-in-the-ass artiste. If you do, you may find
yourself first against the wall come the revolution. And revolutions
go through the publishing business about once every two or three
years, where editors and publishers all leap up and race madly
to grab some other chair, and writers are orphaned and culled
and forgotten. Never forget, publishers, editors, and agents will
not die without you, but you will certainly find your life unpleasant
without them.
Spend time developing new ideas even when you're working
on a book. Keep notebooks, doodle out concepts, create characters
you don't need and don't have any place for yet, write down lists
of titles that sound cool, draw maps to noplace, develop lists
of names. Keep plowing the field of your mind, so that when this
book is finished, stories will already be growing in it and you
will be hungry to write the next one. And the next. And the next.
The writer's work is never done. (But if you set yourself sensible
page limits each day, you at least get some guilt-free free time.)
Remember to have fun. Like what you write, create projects
for yourself that you enjoy working on, don't get cynical about
the process, which is tough, and can be grueling, and sometimes
heartbreaking. Remember to love the writing, and to find pleasure
every day in the fertile imaginings of your mind. Make yourself
laugh sometimes. Don't give up.