I overcome writer’s block by tricking my brain. I tell myself: “All I have to do just write one sentence for today, and that’s it. That’s today’s quota.” I’ll write that one sentence and then I’ll just keep going. I think telling myself one sentence is enough removes the psychological barriers of “I have to write pages and pages and pages.”
Don’t wait for just the big chunks of time to write. Use little chunks of time throughout the day: 15 minute breaks; 20 minutes here; even thinking about your story while you drive home from work counts. Those little chunks of time add up. When the big chunks of time arrive –say a whole Saturday morning with nothing on the schedule– use it. But if something happens and the Saturday morning gets interrupted, it’s okay, because you’ve been using little chunks of time. However, a regular time on your daily schedule –say, either getting up a half hour earlier to write, or writing an hour before you go to bed– can get you in the habit of writing on a regular basis. Then you take those extra chunks of time, both big and little, when they are gifted to you.