The 1000 Foot General blog has a good tutorial on building rocky desert hills that are suitable for a wide variety of settings.
hills
Making Expanding Foam Hills and Rock Clusters
Here’s a nice tutorial on making cheap and easy expanding foam hills for your wargames table.
Making Stylized Wargames Hills
Here’s a good video tutorial on making the sort of terraced wargames hills which are not particularly realistic, but which are eminently practical from a gaming perspective.
Making Modular Cave Tiles For D&D
This tutorial shows how to make modular cave tiles that also serve as hills for wilderness games.