As frank said, be sure not to forget the trash, and the litter. Rusty helmets look great, along with just about any other piece of gear, but remeber to think about what you throw out there. Canteens tend to be well secured, so don't dump them. Gas mask carriers are also always held in a ready position, and aren't easily dropped. In fact, the more I think about it, everything but helmets, trash, and rifles could be found commonplace in no mans land. For trash. I would research the particular army you plan on depicting, and research the disposable things that were thrown over the parapet. Don't shirk away from putting craters either, WWI battlefields usually (not always) looked like moonscapes. Research the battlefield itself too, find out where that tank was fighting. Different battlefields many times had different trench construction.
One think that I think would be important too would be to put the occasional dead body. No mans land was a hellish wasteland, and bodies can depict the true horror of it accurately. The trick here though, is not to do it distastefully, but that is a whole other argument.
Lord Dave
You know you're a modeler when....
a:your fingers are forever encrusted with dried paint/cured glue
b:you smell of enamel thinner, glue, and paint 24/7
c:you shed a tear when the hobby shop closes.
piggy # 76 Lord Stomp and Mig hater