how on earth do you make the wood retainer walls often found in trench scenes out of what material exactly
please help!!

With wood stripes I found at the naval section of the local hobby shop. Another cheaper option is to go at your local fruit & veg shop and ask if they've got a wery tiny wodden fruit box. The plywood with which these are made is very thin (.5 to 1mm) and it's ideal to do the job. To cut it just use a ruler and a sharp knife...
Ciao

What I often use are wooden coffee stirrers. I usually get them from my local coffee shop.

I would prefer the coffee stirrers, they are a nice thickness and width.
If they are not available you can get 'sheet' of balsa or bass wood (basically the same stuff, bass is harder and has closer grains, a bit more expnsive too). You can then cut the sheet down to the correct size. You can also get the bass and balsa wood in thinner strips (depending on your retailer).
The size may be a bit of a situational thing. Depending on the location and availability, the sizes may vary.

thanks all does anyone know where i can buy thin balsa wood im not too sure

I would go with Balsa wood,

Check your local hobby shop, I know we sell some at ours.

~Chip :-)

Usually you can find balsa wood at the hobby shops who carry wodden ships or planes...
Ciao