A piece in The Daily Beast:
“Focus on content, not word count. What matters to me most is that I write every day, not how much I write. There have been a few days where I’ve written only one or two paragraphs. Quantity will take care of itself as the streak builds.”
I’ve also found this book helpful.
peakmemory 
This is good advice, but in my own experience, I’ve found that a modest word count (like 500 words or one single-spaced page) is a very effective technique for getting me to write about “content” for that day