With deadlines approaching, marketing to stay on top of, quotas to fill, and plots to…well, plot, it can be hard for a writer to remember that there is one thing you must always take care of first.
I don't know about you, but when I'm tired I spend a lot more time staring off into space, or writing crap that I end up having to do over again, or making poor plotting choices. So make sure you are getting enough sleep whenever possible. And be honest with yourself about how much is enough—this is different for different people. My husband needs four to six hours of sleep a night, with occasional twelves on the weekends. I need at least six hours every night, preferably eight, and ideally no more than nine or I get headaches.
Also, remember to get off your butt every so often. I know, so much of the hard part is getting your butt behind the keyboard to begin with. But regular exercise, even if it's light exercise, can help keep your brain functioning at its best. Plus, sometimes just the act of getting up and moving around can "jog" your creativity.
Don't forget to eat. I'm hypoglycemic, so you'd think that I'd notice when I go too long without food. Not so much. Particularly when I'm distracted with cerebral activities I can go eight to twelve hours without getting hungry. (Or, maybe more accurately, without noticing I'm hungry.) It makes my husband very sad when I do this, because inevitably by the time I do notice, I'm already into the cranky shrewish stage.
Learn when and how you neglect yourself, be honest about it, and take steps to make sure you take care of yourself.