Get up from the computer!
Your body needs circulation, your muscles need to move. Make a point to get up and walk around at least once an hour. Obviously if you are in the midst of an amazing scene and don't want to interrupt your "flow" you might delay standing and walking, but don't go days of little to no movement while doing a several hour stretch of writing.
Reach for healthier snacks.
I grew up with a computer in the house, and with lots of boredom snacking. It's hard not to sit at the computer and not reach for a snack, and typically an unhealthy one. If you associate the computer with snacking, consider 1) are you really hungry? and 2) can you grab something healthy to eat instead of cheetos and mt. dew? Don't let your love of writing help you to unconsciously feed into mindless eating.
Opt for healthier coffee shop drinks.
If you happen to work weekly (or even daily) in a coffee shop and you order the largest sugary-est most caloric drink they have on the menu, you are doing yourself no favors. Try something with skim milk, no added sugars, or even order a water with your drink so that you can cut your sugary drink intake in half.
Hit the streets.
Consider taking a weekly or daily stroll. This will help get you active, and offset some of the time you spend sitting at the computer. It will also help fuel creative thoughts as our brains are often wired to produce new ideas or mull over old ones when our bodies are doing something else (i.e. sitting in the car, showering, trying to sleep). Walk your own street, or find local parks that will offer you different surroundings to fuel creative thinking. Taking a walk and making it a weekly habit can also re-energize your body and your mind and help you be more focused and excited about your project when you return to the computer.
I've also heard good things about standing desks, and about treadmills that sit under a desk, but I've tried neither of those and so I cannot personally recommend them. Just as writing is a personal journey, your own health and well-being is also a personal journey and you have to try out different changes and see what works best for you.
We all have excuses as to why we let our health slide but just as you wouldn't let excuses keep you from writing, don't let them keep you from staying healthy. In the end, you want to have many years on this planet writing a million books for your adoring fans even if it means taking a walk around the house once an hour for every hour spent in front of the computer.
