Recently while playing games I have noticed that there have been some fantastic times had on themed tables. That being where all the terrain pieces have been picked because a story can be told whilst fighting across it or through it in some cases.
Some ways of putting a theme to your terrain is by making it simply matching your game board ( or mat). Other ways would be to set up a ruined city with mostly building terrain and ruins, with ruined barricades. You could even set up roadways into the area and such.
Another way is to set out low lying terrain, like hills, smaller outpost buildings and forest... this could represent an Agri-world or primitive civilization that has been caught in the midst of battle.
Terrain can be as simple as foam hills, or green cloth set over books, use household materials to manufacture your own terrain, like generators, or buildings. Cereal boxes, foam core and Styrofoam packaging from electronics can make great pieces or terrain.
Battered and destroyed models can also be converted into battlefield terrain pieces, and can look fantastic. Or you can go the route of buying terrain from a varied assortment of companies like Games Workshop, Armorcast, and a few others I cannot remember the names of. Some come painted and ready to go right out of the box, alternately you can build the terrain kits and paint them to your needs much like I am doing at the moment. One of my goals for this year is to paint all of my mainstay terrain projects that I have started, starting with this one :
This is what I have been able to accomplish thus far: