The "bit" turns to days and days turn to weeks and before they know it, they haven't written anything important except their grocery lists. Raise your hand if you know what I'm talking about. So what's the answer? I believe there are a few things to consider.
I've faced this quandary myself. I work full-time, freelance full-time, have a newborn child, am enrolled in a full-time graduate school and have decided to self-teach myself the guitar. Believe me, I know what it's like to feel the pressures of time. And even though much of what I do full-time is writing, I'm certainly not always writing that "great novel" that's on my heart.
One of my favorite quotes (which I can't verify for the life of me) was (I heard) by Albert Einstein. He said "10 minutes a day is all it takes to become a genius at anything." Sometimes I feel I live by that, especially when it comes to doing the things I really want to do. I know I can't spend hours a day working on my great American novel, or I'll never do the stuff that pays the bills. That's ok. So, my goal is just 10 minutes a day.
You might wonder, Can you really write much of anything worthwhile in only 10 minutes a day? Try it! I've actually finished more than I expected in that amount of time, so long as I just start writing. It takes time, but a little effort taken often goes a long way. And, hey, I'm doing better than
the guy who's not spending any time writing at all.
It also helps to keep your enthusiasm up. Become an active member of writing blogs like this one. Read writing magazines. Attend writer's conferences. Read! Anything to keep it fresh. It's not that expensive, either.
I also recommend setting small goals. I try to set three realistic goals each night for the next day. Just three. Then when my day is finished, as long as I've done at least those three things, I feel I've accomplished something. And I always make sure one of those ties into my long-term goal.
And, finally, always take at least one day off. At least one day each week, I schedule nothing. I don't work. I spend extra time reading the Bible and praying. I play the guitar for fun. It keeps me fresh. I encourage you to try it.
Anyway, I don't have it all figured out, but those are the techniques that work for me. If you try them, I hope they work for you too!