Personally, I would like to keep myself hydrated at all times and keep my back up straight. Long periods of fixation towards computer monitors could really dry up your eyes and cause fatigue. Definitely take breaks to relieve your body at times.
One more piece of advice is to take a break after every 2 hours. This will help increase productivity. I noticed that after spending hours in front of your pc and you do not take a break, you'll end up achieving nothing. So get to know how long you can concentrate and go on a break when you reach your limit.