Write first, edit later.
Don't be afraid to make mistakes...your first draft doesn't need to be perfect.
The only thing you can't edit is a blank page.
However you say it, in my opinion this is one of the most important pieces of advice around! When I write, I still find myself reading over the previous scene and tinkering with it a little before I move onto the new scene. But I've forbidden myself from going back to Chapter One and starting revisions UNTIL I've finished writing my last chapter. That way lies disaster for me - I can't count how many manuscripts I've given up on without finishing, simply because I got caught up in revisions and failed to finish the first draft.
And in my next novel, I have high hopes I'll even be able to resist tinkering. Wish me luck!
How about you: do you revise as you go? Do you follow this advice religiously? Or have you found another way to make things work?