Don't edit, rewrite, criticise, correct, change, just write
This is a very hard lesson to remember. When I read what I have written, I cannot stop thinking whether I can say it better, using fewer words, whether it sounds good... I always, always, always start changing what I have written, rewriting, editing, correcting, changing... which means I'm not writing anymore. Which means I'm not doing what I am supposed to be doing. I am disrupting the flow of writing. I am not thinking about the story anymore, but grammar, style, impressions...
I must remember that there will be time for editing, and it is not now.
I also keep jumping back and forth because I forget some details, like last names, first names, who's who and what color it was. That too stops the flow.
Another thing I do is try to find "right words". English is not my mothertongue, and my vocabulary isn't that good. I tend to think in Finnish and some in Swedish, and some times it's only the French word that comes to mind, or some other obscure language. I should remember that I am supposed to WRITE now, so I'll put in which ever word that comes to mind, or leave a blank, if I can't think of a word, and change that when I'm editing the text. That is, NOT NOW. I am supposed to write the first draft, for heaven's sake, and that is NEVER, NEVER, NEVER perfect. It isn't supposed to be either. I need to have text I can edit.
Also, check the details later. Check the facts later. Don't do the research while you are writing. Just write!
About the research: all the major parts should be clear BEFORE you start writing. You can check the minor details while you are editing, or after the day's work. One may not take time off the allotted writing time to do this kind of things. One may just write.
Don't allow anyone else to critique your writing either. There is a saying in Finland - unfinished work should not be shown but to lords and loonies. (Because neither accepts a no as an answer... and their critique is not worth listening. ;-)) But seriously, I have made that mistake a couple of times, and it kills the inspiration.
Elizabeth suggests that one takes the day's text to bed to be read the last thing one does that day. Sort of a reward. One may have a red pen and mark the parts one thinks should be changed, typos and such, and one may write notes in the margin, but that's it.