I solved my problem, eventually, by changing my goal to fewer words. Now I consider that I have written if I do 200 words. An interesting byproduct of changing my word goals, though, is that all kinds of time that didn't use to feel like it was available opened up. Two hundred words, after all, can take just ten minutes if I get going. Every ten minute gap in my day, then, is potentially the entire day as far as writing is concerned. I went from thinking that I couldn't write unless I had a lot of uninterrupted time, and that my day's writing had to happen in one session, to realizing that I had multiple opportunities in the day. By lowering my goals, I raised my production.
Another interesting effect of lowering my goals and using the cracks in my schedule to write is that I write faster if I have both a beginning and end time in mind when I start a writing session. For example. I get to school everyday at 5:45. I plan, grade, run off copies and finalize my lessons until 6:30 or so. School doesn't start, though, until 7:25. What I've been doing is writing during that time. Only 40 minutes are usable (students start showing up), but they are a good 40 minutes because I can't afford to sit at the keyboard daydreaming. I need to write.
In Zen in the Art of Writing, Ray Bradbury talks about writing Fahrenheit 451 on a library typewriter that sold him 1/2 hour of typing time for a dime. He'd plug the dime in, then type like a mad man for 30 minutes. The time constraint worked in his favor.
Writing in pinched spaces can be a great technique. On days when I actually have open-ended time, I'll give myself artificial time slots to fill. For example, I'll start an album on my media player (the sound track to The Natural was my accompaniment Saturday), and then see how much I can accomplish before the album ends. If I'm really sluggish, I can do the same trick with a single song. The idea is to encourage words on the page.
Staring into the distance has its place, but too often my staring isn't about working on the story, it's about not writing. Writing in constrained time periods helps me. Being scheduled helps. If you are worried about your own productivity, or you have a lot of days where you don't write, or you sit down to write without getting much done, you might try putting yourself on the clock. It could help.