First, you gather your materials. I use pencil for grading (except for very short journal sorts of stuff, for which I use different colored pens), but I also often need coffee or tea, some music (I prefer classical or jazz without words), and a fairly solid notepad.
While gathering your materials, you may become distracted. Perhaps there are dishes in the sink. Or maybe there's effbee. Or snow to push around on the driveway. There's probably something. Fight the urge.
You sit down, pick up the first paper, and start to read. Congratulations, you've just done the hardest part, the getting started part. If you're lucky, the first paper you've read is good, and you can power through several more. If you're not lucky, and the first paper sucks, then the dishes become so much more important.
No matter how brilliant the assignment, how good the papers, you power through a few, and then you want a distraction, something else to do. So you count the papers. You've done six, and there are 30 more to go. This is the vital moment, almost as difficult as sitting down to the first paper. If you can power through just a few more, then you can get to being a third done. And a third done is close to half done, right?
If you can't, then may I suggest writing a blog post about how difficult it is to power through?