Find your worst enemy and learn how to deal with it. Mine is sun and heat. Learn to rest with eye protection on and your eyes closed or even sleep if you can under any conditions. Lying on a tarp or a ground sheet is still better than sitting in the trench with eyes wide open when you can be resting. Don't wake up and join the crowd when you hear other guys chit-chatting and you don't really feel that tired at the moment unless your are ordered to do so. Use any chance to rest, even if it's a 15 minute nap. You are not taking fire or being a part of active patrol/guard duty, no reason to be on your feet. I noticed that people have this tendency of refusing to rest when they have a chance and going out, talking to guards/people who just got up for a patrol etc. Our boss fixed that issue pretty fast during WW2 event this summer. We woke him up at 6am, he got up and said: " I see you are all rested and don't want to sleep, well, grab your e-tools and go fortify/add camouflage to the trenches than"
Don't go all crazy and pumped up on energy drinks/caffeine/sugar in the first 5-6 hours like some people do and than drop from exhaustion. Water/food is also very important like many people already said. Take a drink every time you have a chance and refill every time you have a chance, even if your water is not low. Worst case it'll come out as sweat. Still better than dropping down from dehydration. If you are out of water, ask someone instead of going without a drop during long patrol. I don't use any power bars, energy drinks, Gatorade etc but that's just personal preference. Gatorade did save me once from being completely sun-fucked but I just like old school natural food/drink. I usually bring smoked sausage/meat, dry cheese, any kind of good bread from European bakeries and instant mashed potatoes, sometimes hard boiled eggs and canned meat. Meat and bread can be easily consumed in the field, will not go bad even on hot days, way healthier than MRE's and won't give you constipation
That's basically all the food you need to survive for a few days. That's what real soldiers used for hundreds of years, so it must work
To make potatoes all you need is 15 minutes, some water, mess kit and a portable Esbit stove which is the same size as your regular pack of smokes... Having some hot food on a cold day/after you wake up is always a good thing. I usually make that during rest times. Brian McIlmoyle likes to treat guys with hot cup of espresso or regular coffee in the morning
That thing will wake you right up. It's also a good idea to know your body, it's biological clock etc. I am for example a night person, so I try to take watches/go on patrols between 9pm to 4am than rest for an hour or two if possible. Plus I noticed that most people usually become less active/more sleepy during early morning hours. Between 3am and 6-7am. Like other guys already said, make sure that your gear is prepared for a sudden wake up call. Before going to rest reload, refill water, eat, make sure you can put your webbing/rig on within a minute or at least make sure that you can shoot back. At least have a rifle and 3-4 mags within arms reach. Always know where you gear is. Lots of times people come back to the base camp tired, drop their gear, than crawl under a poncho or go down on a blanket and sleep while gear sits 10 meters away. Not good, especially at night.