Trench warfare was new and confusing in World War I. The militaries didn't realize how advanced warfare had become, and it showed in there tactics. Armies just lined up and attempted to walk across the battlefield. They quickly realized that these tactics will no longer work and they stopped using them. To adapt to the new warfare soldiers should desperately want these five things.
When out in middle of no mans land a shovel is mans best friend. A shovel is the most effective way to get out of a jam. If you get stuck in no mans land the best thing to do is dig yourself a hole with your handy dandy shovel and your immediately better off because you are covered from enemy fire.
Disease was very common in the trenches since there was very little way to sanitize yourself. In the trenches there was no way to rid yourself of feces there was just a corner where everybody vacated their bowels, and they wore the same dirty clothes everyday. To help keep yourself from catching disease from the dreadful conditions rubbing alcohol could sanitize your hands and keep you healthier than the rest.
To put it simply water is a necessity. Both sides of the war were idly locked in their trenches for three years until one could figure out trench warfare, but in that time you couldn't leave or else you would be killed for desertion. So in that time many people become dehydrated and unfit for war because of it having water consistently is an automatic advantage.
For the most part in the trenches nobody had a change of clothes. This led to a lot of sickness because the uniforms would accumulate dirt, feces, urine, etc. An extra pair of clothes can save your life and at the very least it would make life in the trench much more tolerable.
When your stuck in a trench for three years you get a little bored. You need something to pass the time. Without anything to keep your mind sharp and busy you could go insane. Cards help occupy your time and keep the day moving along.