I have to admit that my main method of dealing with multiple writing projects is not to have multiple writing projects. I am very bad at thinking about more than one thing at a time. Whenever possible, I have a single item on my list of things to write, or I work sequentially on projects, putting one to rest before I start something else. Of course, life sometimes devises other plans for me and I have to think about more than one thing at a time.
So my advice is first: don't do it. Secondly, if you have more than one thing to write, order them sequentially and get the smaller projects out of the way as quickly as possible to get back to the main enchilada. Never spend longer than a week on a book review or a peer review. The main enchilada is always the dissertation or the book project.