For me it was advice from Dan Harmon: “Don’t try to prove you’re a good writer, you’ll never write anything. Try to prove you’re a bad writer and you’ll write everything.” Not perfect advice but it really does help me write when I’m being overly critical of my ideas.
Got many advices along the years that have helped me plenty, most have been posted already though. One advice I got along the way that I’m fond of is that your writing should always try to say something. Your truth, your message, your feelings. This one is more aimed at fiction, of course.