Incidental terrain are those small bits of bush, rock outcroppings, and other features that serve to break up the look of a flat gaming table. The Battlefields and Warriors blog has some tips on creating battlefield incidentals. I really like what he has done and realize that I don’t have enough on my own tables.
Dux Humunculorum has a post on making magnetic terrain. The idea is to keep your bases from slipping off the slopes.
Although this video is focused on Warhammer 40K, I think the techniques shown for building terrain entirely out of cardboard are worth studying.
Hills are among the most basic sort of terrain you need for a wargames table. Here’s some nice advice on building inexpensive cardboard hills.