Cory Doctorow's article from Locus really nails a number of issues that apply to the writing we do in class.
Short, regular work schedule
When I'm working on a story or novel, I set a modest daily goal — usually a page or two — and then I meet it every day, doing nothing else while I'm working on it. It's not plausible or desirable to try to get the world to go away for hours at a time, but it's entirely possible to make it all shut up for 20 minutes.
Don't be ceremonious
Forget advice about finding the right atmosphere to coax your muse into the room. Forget candles, music, silence, a good chair, a cigarette, or putting the kids to sleep. It's nice to have all your physical needs met before you write, but if you convince yourself that you can only write in a perfect world, you compound the problem of finding 20 free minutes with the problem of finding the right environment at the same time. When the time is available, just put fingers to keyboard and write. You can put up with noise/silence/kids/discomfort/hunger for 20 minutes.
If we're to teach writing as a skill, it's the doing of the skill that matters.