We all have days when we write that I refer to as throwaway days. Days where my prose is not flowing in the way I would wish or my research has yielded nothing despite the hours I spent looking for something. Sometimes you have to admit what you started doesn't work and have the courage to leave it behind and start again. In terms of writer's block it always effects me at the very start of a new work, I do okay as long as I am in the middle of a piece with threads to pick up. So when that happens I usually walk away from the screen and start to write a list of things I want to cover overall in the piece or I diagram it in terms of how it might be laid out. I find that a lot of times I change my introductory paragraphs so that helps as well, to know that what I am writing to start is likely to be changed, that way its not so serious. If you are really stuck just imagine in your mind what you would tell a friend about what you are about to write, if they suddenly called you when you felt blocked. Writing a pseudo response if you like to them usually gets the ball rolling. Lastly, always have notes before you sit down to write. I do a lot of notes around my research and that means their is always something already started beside me, I am not alone with the blank screen, I have my cheat sheet with me!