A teacher of mine once gave me some advice that has summed up my approach to writing--sloppy writing is generally the result of sloppy thinking. So if your thoughts are clear then your writing will generally get clearer.
I thought I was going to do a lot more writing than I did but what I ended up doing instead was a lot of thinking. Thinking is the actual, important part of writing. So I think I've worked out an approach to a new writing project that will let me approach it in a sensible way over the long haul.