When you throw yourself into a big old writing project you'll find it considerably easier to forget that you have a body (unless, of course, you have arthritis/beginning carpal tunnel like I do). That's not a good thing. It's less than good. It's just bad. Now, aside from resting and stretching for the things you stress out during writing (back stretches, wrist stretches, eye rests, etc.), you need to work out your other bits of body, too. It seems really hard because all you want to do is curl up with ye olde laptop and type out your ninja lesbian steampunk stripper romance.
Resist the urge. You need to make sure you do something for your body. The easiest way I've found is, since we're so obsessed with getting to our writing, make yourself do your exercising before you can touch that manuscript. Whether you go walking, hop on an exercise bie, do twenty rounds of shadow-boxing, or hop on the DDR pad (like yours truly), make sure you do it. I reccoment anywhere from half an hour on up, but even five minutes of exercise is better than not doing anything.
I've also found that it's quite a bit easier to fix your diet a bit if you have at least a month-long project. Bored eating is a huge thing when it comes to overt eating. By having this project to fall back on when you're ored you can stop that binge eating in its tracks. Now, that's not all it'll take, mind you. You'll still need will power to change this drastically, but having this project is not going to hurt your chances.
Also, you can change your snacks up a bit. Grab an actual strawberry (they're really good this time of year) or slap a Pink Lady past your gums. Don't do it a lot, but...well, you know how it goes.
Aside from all of that, changing your diet will make you feel better and write better and the exercise will give you plenty of time to think through your story issues. Besides, repetetive physical activity frees up your brain cells.
Be merry this month and keep going strong in whatever writing you may or may not be doing,