Now this is a really important one. I don't know if it's that I'm just getting older, but I just can't sit as long as I once did. So, I have gotten into the habit of setting an online timer for one hour and when it goes off, I get up and out of my chair and go for a walk around the house for about five minutes. When the warm weather comes, I just might up it to a walk around the block... but it's cold today! The important thing is to get moving! Do something to loosen up and then go back to your writing.