Willpower is the number one thing. Sometimes you may not feel like you are doing anything worthwhile with it. The initial excitement died and real life pushes it away. You start making excuses not to write and pretty soon it sits forever unfinished in your hard drive.
There are days when I really have to force myself to write. I usually reserve my fiction writing for the end of the day, when the kids had fallen asleep and the house is quiet. When I'd rather lay back and relax watching my new crush Josh Gates on "Destination Truth", I instead fire up my netbook and continue the journey of my fictional characters.
Another thing that keeps me moving along is that I promise my test readers that I will provide them chapters each week of my work in progress. I'm am on the second novel that I've done this with. Reading the reactions to my characters and story lines gives me not only valuable feed back but also a kick in the tail to keep going. I don't have it in me to leave those readers hanging. Now I realize this method isn't for everyone; you have to get passed the fear of letting others see your finished work before you can even think about passing out a rough draft version.
Don't get me wrong. I don't write on my fiction every day but I do write on it most days of the week, even if I only have time for a 30 minute session.
Most of all, it's about persistence and dedication to your goals. Here's hoping you virtually dust that old unfinished novel off and get those characters moving again!
Peace and happy writing,