Cardboard is not the greatest material for making wargames buildings — formcore is far better. But for making ruins, it actually works quite well. I built enough ruins to cover a 5×5 table for a Stalingrad game from cardboard. Terragenesis has an article on making ruins from cardboard that look remarkably like the ones I made.
modeling
Modeling Walls and Barracades
Here’s a nice video on modeling walls and barracades for wargames. The techniques are particularly post-apocalyptic.
Making Casualties From Clicky Figures
The Combat Zone site has some instructions on making casualty figures from those collectible clicky figures.
Making A Barge From A Date Tray
Barges — of the sort used to transport troops and supplies — can be used as central pieces in games set in a wide variety of periods. Here’s a set of instructions for making a barge from a date tray.