For those modern and near-future games, here’s an illustrated tutorial on making a really neat shantytown.
Modeling
Making Concertina Wire
Here’s a great tutorial in making some very realistic concertina wire for your 25mm – 28mm miniature games.
Making Terrain From Cardboard
Although this video is focused on Warhammer 40K, I think the techniques shown for building terrain entirely out of cardboard are worth studying.
Making Tabletop Rivers
The LudStuff blog has a nice essay with photos on making tabletop rivers.