Isolate your project from everything else. Have a writing to-do list with only one thing: writing the thing you need to write. Figure out how to clear away other agenda items that are getting in the way.
Suppose you had a to-do list with 10 things.
(1) Grade papers.
(2) Go to the supermarket.
(3) Finish the article.
(4) Take a shower.
(5) Answer email.
It would be pretty easy to get everything done except for (3). You would have been busy all day, too. But being busy is not the same thing as being productive.