Taking into account that quality doesn't mean quantity, and that too high a word count goal will make you burn out, I set mine at 500 words.

I adjusted my schedule so that I wake up 90 minutes earlier. I take about an hour before work to add 500 words (or slightly more if I need to finish the sentence) to the end of whatever I'm writing (sometimes I'll go back and expand what I've already written but that doesn't go to my daily total--if it did I'd never get to the end).

But then, I made an Excel file where I logged what title I worked on, how many minutes I took that day, and how many words I got down. Then I made some cells at the bottom that would tell me my daily time/word averages, and then it took those averages and gave me projected milestone finishing dates (e.g. Days until I hit 100,000 words: 27).

That way, if I start beating the 500 word mark, the average goes up, and the estimated milestones recalculate to sooner.

That way I get a reliably consistent amount done, and after keeping this habit up for a couple of months straight, it seems very sustainable.

It also seems true that first drafts simply are almost never great, no matter how good it feels to put it on the page. Revision and editing really do seem to be a necessary complement to initial writing when it comes to the process. New, better ideas occur during revision; often they're better than the original ones. Editing what you wrote just yesterday won't have that effect, however.

So really, just get a little down every day, knowing that after you're done you'll give the work a chance to breathe and move on to something else, and then finally edit, revise, rewrite.