Take a break, regularly. This is an ongoing topic for me. This morning I was doing some creative work but I wasn't really in the flow. Instead of leaving the desk I kept going and felt even worse after a couple of hours. And guess what I didn't come up with a great design either. Lesson learned: I go for walk with my dog now :) and start using a timer that reminds me to take a break for 5-10 minutes every hour.