I have that problem sometimes, too. What I do is I get another document open and write down my ideas for what I'm about to write either in the order of my thought process or the order I think it would be best in; since this document isn't the final one, it doesn't matter how it's arranged. Then, I can worry about putting it into perfectly manufactured sentences in the actual essay.
Another thing my mom says to do is to turn off the monitor and type it; that way, you get the ideas down. Then, you can look at it and arrange it how you want. Of course, this only works with a desktop.