I can only tell you what works for me. Don’t focus on your current project for a period of time. Let’s say a week. Watch your favorite movie. Then watch your least favorite one. Compare what you loved about the former and hated about the latter. Sometimes, just thinking along the lines of “what a great line, I wish I had written that” or conversely “what a horribly contrived situation, I could have done better” is enough to spark a renewed energy. Hope this helps a little.