Lee-Enfield rifle by jnzl's pictures, creative commons license
This is the rifle of the British, the Lee-en field .303. It has great accuracy for sniping across no-mans land. I also recommend a bayonet attachment.
Crochet chevron blanket by twilight taggers, creative commons license
In the trench, you're going to want at least 1-2 blankets. While your on break, you need your sleep. Winter is coming, and you need to be prepared.
US Military Case XX V-24 by Micheal arrighi, creative commons license
Bowie knives are handy, it can be used for cutting things like rope, your not going to try and cut it with a bayonet. It can also get you past an enemy in close quarters, if you have a loaded gun with bayonet, use it. If your gun is out of ammo, or broken, use the knife.
Gas mask of WWI by quinet, creative commons license
The Germans on the enemy lines are going to use gases, mustard, chlorine, etc. Don't think you can just run past it without breathing, you need air to run well or your muscles cant move. It gets in your eyes, your pores, every nook and cranny, point is, wear a mask every time.
...or what's left of it. by cell 105, creative commons license
A first aid kit is practically required. You might not get hurt, but someone will, The shrapnel and artillery can do some damage. Make sure it always has gauze or rubbing alcohol, and a lot of bandages.