24 hour games are not as hard as everyone thinks if you are prepared. It's also the most rewarding and fun type of airsoft you can play (in my opinion)
The biggest thing that I see that flat-lines people is not getting enough food, water, and rest. New players tear into the game and play flat out for the first 6-8 hours without resting or eating enough and are either bagged for the rest of the game or pack it in when they realize that they are only a third of the way through the game. Add to that that once they get wet, either by sweating from working so hard or from the weather, they get cold and the problems get worse. Wear a wicking layer underneath your BDU's even if it's hot outside. When you sweat the poly layer will absorb your sweat and actually make you cooler through evaporation. If it's cooler out, it traps heat next to your body keeping you warm. It also dries super fast so that when night comes you can ditch your BDU top if it's wet and put on a fleece and be dry and warm with two layers. Buy good clothing, it's hella worth it. It's also nice to have an extra set of BDU's if the weather sucks and/or if you have room. Once you stop moving, you'll cool down quickly so have a ranger blanket or something to keep warm. Also make sure you are bringing a tarp or something water proof to either keep the rain off of you AND your gear or to put down to keep the dew from getting stuff wet. We usually bring one tarp for every 2 or so guys to share.
I usually plan to have a MRE sized meal at least 3 times in 24 hours with protein or energy type bars in between when I feel munchy. I usually have at least a few bars on my first line that I can munch on when we are taking quick breaks and relaxing. When we have our big meals our squad usually takes an extended break to remove all of our gear and get dried out if possible. You should feel great after eating and rehydrating and if not, maybe you need a bit more of a break. I usually regulate my hydration is by making sure that I have to take a leak every 2 or 3 hours and if I don't I drink more. It depends on how hot it's going to be, but I usually plan on drinking 3-4 liters in 6 hours during the day, less at night.
If you will be playing as a squad or with other guys you know, get together and try to elliminate all of the duplicate things you need. Not everyone needs to bring pistol gas, a set of tools, burner for boiling water, or baby wipes (the greatest thing ever for a tactical dump) one per group of guys is good.
Batteries are totally dependent on what gun you are running and how much you plan to shoot. A rough rule of thumb is one bb per Ma/h on your battery for a stock gun. I usually have 2 batteries on me and have never gone through my second. Again, another spares thing you can work out with people you play with.
We also tend to bring an extra gun for every 3 people in our squad. It sucks to have your gun go down in the first few hours of the game.
All of your crap should fit into a 3 day pack with water being the biggest pain. Alot of fields will have water either provided or it will be available.
Definitely the most important thing to bring is a good attitude. You WILL get tired, hungry, cold, wet, angry, frustrated and it's important to yourself and the players around you that you understand that it is all part of the game experience. If I think of the best times I've had in airsoft they've almost all occured while milsiming -- sometimes while palying in the most difficult conditions.