When you're sitting down with the intention to write, do not, under any circumstances, go back to what you wrote the previous day and edit it... set aside time for editing, but when it's time to actually move on with the story without looking back at that typo you made in that paragraph you wrote yesterday, do just that. Otherwise - no progress. Even if you end up with the perfect excerpt, the low word count can be really discouraging, especially if you're attempting a novel or something of similar length.