From an email that I recently wrote to a student suffering from writer's block:
Have I told you my story about William Stafford, the poet? He made it a habit to write a poem every day. (A great poet, he won the National Book Award, etc.) Anyway, I saw him read his poetry shortly before his death. A budding writer stood up after his reading, during the Q+A and asked, "You said you write a poem every day. What happens on the days when you're not feeling inspired?"
Stafford replied, "I lower my standards."
I think that about sums it up.