My wonderful agent, Lucienne Diver, has been answering questions this week at the Absolute Write Water Cooler. You can check it out here (though you’ll have to scroll over to Page 2 for the beginning of her answers).
Scalzi on writing fast here and here: I find the most important metric for writing is whether I’m happy with what I’ve written that day. If I am, I’ve written the correct amount, regardless of how many words that amount ends up being. I think this is a good guideline for writers.