I am currently building a W.W.1 dio (very slowly lind you!) ,here's a few pics of some of the techniques i used up until now.
I went for an Emhar tank, the fitting is a nightmare but it isn't the worst kit out there..
Here's a base that is basically what Scott suggested but with photo's.
Base with styrofoam:
Now i had the same problem as you,i had no idea what the walls of the trench might of looked like, except for a few reference pics.I went by this one:
And improvised with some old speaker wires entwined around some wooden sticks:
Instead of celluclay i went for paper maché layered with plaster:
After seeing various photo's of other dio's i came across alot of corrugated iron used in defensive positions such as trenches.Some sticking dangerously out of the ground, others as shelter.
I simply removed the backing off of the flap of a cardboard box and spraypainted to seal.
This is the stage i am at now during the painting stages:
This isn't the way TO do things, it's just the way i did things.Other guys will hopefully contribute to give you some more advice.
Here is a great W.W.1 dio example:
Some handy reference links of various photo's from W.W.1 tanks and battlefields: http://www.wwi-models.org/Photos/index.html http://www.wereldoorlog1418.nl/warpictures/index.html
Good luck mate,keep us updated!