A sure way to prevent this is by committing to it every single day. Tell yourself that when you wake up, no matter which day of the week it is, you’ll write for 30 minutes. Adjust this number accordingly; the most important aspect is that you actually do it. Don't get swept away by the flow of events in your day; it's too easy to surrender to life's mishaps. Schedule time for yourself, and make it your personal crusade each and every day.
Aside from carving out the time, I'd also recommend setting an explicit word goal; this will prevent you from staring at the same sentence for the length of your writing session. The length should be a moderate, perhaps 500 words, depending on the time allotted. You can also be aggressive—whatever works for you—just don't set an impossible goal. Remember, the word count must be achievable and repeatable. Set a reasonable goal, and if you think it's still too low, add 10%.
It would be great if all of us could write 1,000 words before the day unfolded, but for many this isn't possible. Most of us have jobs and families that keep us up to the wee hours of the night. But consider for a moment what you could achieve if you found a way to make it work: if one were to write 1,000 words a day, in a year's time they'd amass over 365,000 words (the equivalent of 5-6 short novels). I’ve heard of writers waking up two hours early just to squeeze in writing time. Are you one of these people? If necessary, could you adapt?
One of the greatest challenges isn't scheduling the time or finding a quiet, uninhabited space--it’s getting to bed at a decent hour. Let's face it, by depriving yourself of sleep, you instantly deflate your brainpower; and trust me, you'll need every brain cell when writing. In setting your schedule, don't forget to select the appropriate bedtime. This small consideration will go a long way towards ensuring your success.
Don’t forget to arm yourself with an interesting topic to write about. Writing in your journal is only so effective. Over time, you'll find yourself rehashing the same ideas over and over again; or worse yet, writing about writing. Use those precious moments to work on an actual project.
For me, I found that breaking a novel into smaller chapters (a la James Patterson) is quite effective. In this format, each chapter consists of 500 – 1000 words. As I labored over the first draft of my novel, it was easier to keep the momentum going since all I needed to produce was a few pages; in no way did I feel overworked or overwhelmed. In a couple months, I had an entire manuscript in hand. Not too shabby. And it was an amazing experience, especially after I proved to myself that I could be productive when I dedicated myself to it.
Choosing to write early and often is a small tax on your day.
In closing, productivity stems from a starting point; so by all means, set your anchor and extend the lines. You don't need to be a great author with exceptional time management skills--just a consistent one. The rest will come with practice.
Never be afraid of sticking your toe in the water. Dive right in!