Mercerius Atticus has created some very impressive flat buildings. This isn’t a how-to, per se, but the idea is worth passing on. These would be especially useful on the edges of a game.
Matakishi has some instructions with photos for making World War II era buildings out of sheets of cork.
Here’s a clever method for making false building fronts using molds created from craft store sheet foam.
The Alternate Realms site has several nice cardstock buildings, including some huts and a chapel. The page also has a couple of tanks and a science fiction walker vehicle. Finally, there’s a model of St Michaels tower on Glastonbury Tor (sp?).