The theory behind my pitiful target of 250 words at a stretch (and they don't have to be wonderful words, they can be, and often are, utter drivel) goes back to a book that became my Bible in my depressive mid-twenties. Feeling Good by David D. Burns (otherwise known in my circle as "The Yellow Book") was one of the early texts on cognitive behavioural therapy. Essentially, it argued that your thoughts can influence your feelings, and therefore can be to some extent consciously managed; this was a bit of a revelation to me and my depressively-inclined mates.
Anyway, one of the many useful pieces of advice that I gleaned from its pages concerned the paralysing effects of perfectionism. Better to aim very, very low, and achieve, than to aim high and fail. As anyone who has been depressed knows, there are times when any activity at all seems impossibly hard, even getting out of bed. Therefore, better to write one sentence and feel good about doing it than to tell yourself, I must write 10,000 words today - a target you will inevitably fail to meet, and then beat yourself up about.
The magic trick of this technique is that almost always, you find yourself exceeding your very low aim. You say, okay, I'll write one sentence -- but before you know it, one sentence has become two, a hundred words becomes 250, and then a page, and the shot of amazed pride you feel in your achievement surfs you onward almost without you noticing it. Once you get started, it is so much easier to keep going.
Doing anything, however small, is better than doing nothing.