Leo Babauta at Zen Habits has the lowdown on how to write every day. And it all starts with a reason.

Most important: Have a great reason. The rest of this doesn’t matter if you skip this step. Answer this question: Why do you want to write every day? If it’s because it sounds fun, sounds cool, sounds nice … you’ll abandon it when you face discomfort. If you want to do it to help someone else, to make the world a better place, to lift someone’s spirits, to reduce your pain, to find a way to express your deeper self … then you can call on this deeper reason when things get difficult.

— How to Write Every Day by Zen Habits