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.
Do not let yourself forget that once writing is your job, it is exactly that. 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.