Find an employee that allows you to work from home at least 1-2 days a week if possible, or at least that has flexible hours so you can, for instance, get into the office a 7AM and leave at 3PM. Both these things have given me a lot more writing time over the years.
WRITE IN 10-20MIN SLOTS
If you have 10 minutes and a laptop or phone or notebook, you can write.
Carve out space where you can and force yourself to do the things even when it sucks. I had a 5-hour commute 3 days a week for a year. Every workday I wrote 2000 words hunched over my laptop on the bus. It sucked, but I did it. Now I don't have to commute ever again.
LUNCHTIME IS YOUR TIME
Eat lunch at your desk and go home early to write. Or write during your lunch hour if the culture of your office allows it. I was never at work to become BFFs with my workmates, so I’d always avoid taking time off for lunch if I could avoid it.
GIVE YOURSELF AN END-DATE
Write down the date you're going to leave your day job. Write it in your calendar and let it help you stay sane as you ramp up your efforts.
CHOOSE YOUR CAREER WISELY
If possible, do something for a day job that will help you with your writing career, maybe by working in publishing or teaching so you gain contacts, or in marketing so you learn about how to promote your books.
HACK YOUR PROCESS
Use hacks like the pomodoro technique to make sure you’re making the most out of the time you do have to write.
GET YOUR PARTNER INVOLVED
Talk to your partner/s, if you are into that, often and openly, about what you’re trying to achieve. They can be supportive but also miss you, and it’s good to share your wins and make sure they’re aware that this crazy pace is not forever.
PROTECT YOUR EMPIRE
Think about how to protect your income when you DO go full-time, so you aren’t forced to go back the way I was the first time. Make sure you calculate exactly what you need to survive, and account for taxes, insurance, advertising, book covers, editing, etc. Create a realistic budget.
Make sure you ensure the things you need to insure, and that you've got some savings to tide you over if things take a dive. Pay your damn taxes. Look at freelance work and other ideas to help you even out your income.
BE RUTHLESS WITH YOUR TIME
Develop a draconian attitude toward your day-job hours. If they pay you for 40 hours, they get 40 hours. No more, no less. You’ll put in the effort while you’re there, but any unpaid overtime you do robs you of your limited free time. Remember, you’re not sticking around forever so it doesn’t matter that you don’t win, “most dedicated employee of the year” award.
TAKE THE FASTEST ROUTE
Work on things that offer the fastest route to getting what you need. You only have finite time, so choose to only do the jobs that offer the highest ROI. Publishing a new book is much better than updating your website or making another social media page.
THINK ON THE CLOCK
Do all your thinking at other times of the day so you don’t waste your precious writing time staring at a blank page. I do my thinking either in the shower or dozing in the car or on the bus. Sometimes during boring meetings.
REMEMBER WHY YOU'RE DOING THIS
Stick your goal in the calendar. Glue post-its all over your house. Put an alert on your phone… anything to remember why you’re doing this!