For a wasteland look I use tile adhesive + some rocks or rubble, basecoated with Valejo German Camouflage Black Brown, liberaly drybrushed with US earth and afterwards with Iraqi sand.
Sometimes I even highlight with a little offwhite.
To finish it I glue one or more clumps of static grass (the dried out variety) and/or some Silflor tufts.
You could, ofcourse, add some scenic elements like weapons, body parts, skeletons (like Lead Adventures Miniatures' Dead Stalkers) and whatnot.
For the destroyed urban look I use Fenris' Incursion bases. basecoated 'em black, then drybrushed to light grey (and when necessary I pick out the details like, spent ammo cases or a drain cover)
For pics check out my photobucket 28mm/PA album (link in my sig)