I'm no where near writing a thesis yet, but I often have difficulty with papers...no matter what length. What I have found to work for myself is basically the "just start with something small" bit. Maybe look at it a different way though. How about, just write something, any thought that you have about your thesis. And I DO mean ANY thought. I once wrote something along the lines of "This paper assignment is the most ridiculous piece of %*#&%*@!&% ever!" As I went on with this thought, it turned into an actual thought about the content of the material the paper was to be on, then very rapidly, a body paragraph that worked nicely. I ended up getting an A on that paper.