One of  the areas which can set a scene apart is the detailing of its pieces.
While the versatility of a straightforward "grey building" paint job has its merits, such as quick mix and match into any setting, a custom designed terrain board really comes alive if you take the time to make the elements work together.
Wargaming groups can be a terrific source of information for modeling and terrain building, and I encourage anybody to join some.
One of the groups I belong to "Flames of War Pacific" had a couple of great ideas posted that I wanted to share here with others.
The first is from Royston Boss, passing along some tips from his friend Rodg:
1)  Buildings are part of the terrain in which they sit. It colours them.
2)  Rain runs down from roofs, window sills and doo lintels, and causes streaks on the buildings from the top down. Such things as iron gutters and corrugated roofs cause rust streaks.
3) Rain hits the ground and bounces up causing some ground colour to go up the side of the building.
4) Mould grows where the building meets the ground and where it runs from gutters . Mould is black/grey green.
5) High traffic areas such as dooways develop their own pattern of wear like around handles. A gun slit wears where the gun rubs against it (if it does).
6) There is generally mould an lichen growt  around the base of chimney pots or aerials on a roof and mould etc grows mostly on the Northern side of a building in the Northern hemisphere.
7) In tropical climes plants grow around an on buildings very quickly  and will soon obscure the join to the surrounding terrain. In temperate climes it takes longer, but a bunker built for more than a year will definitely have growth around it unless some drill sergeant is having it  bulled an blancioed frequently.

Added to this isa post by Mal Wright:
The most important thing everyone can do, is to observe nature around you. Take note of the colour of trees and undergrowth. Even if you live in a city there will be parks and gardens. Observe the way trees look. They are rarely a ball of leaves on the end of a stick trunk! Also note the way shadow effects the area underneath. You can make them look better by adding dark colour beneath bushes and trees. The floor area of woods should be much darker than surrounding areas.

Take note of how weathering effects buildings. Look at some being demolished and note that gravity rules. Rubble falls downward and collects where it fell. Damaged bits hang down according to the support remaining, and the gravity effecting their weight. This is an important instance where gravity has a counter to its pull.   Look at how real roads sit on the terrain they pass through. There are always drains and sometimes these can be substantial. Model roads sitting on a table top terrain often look wrong because they don't take drainage into account. Drainage causes the growth along the edge of roads to be thicker and darker in colour. Even if now dry, moisture that gathered at some other time will have drained off and effected the edge of the road.   If it is a dirt road the moisture will have tended to gather in wheel ruts and hollows. These will be darker as a result.

A touch of dark brown can make them look good. A touch of dirty greenish brown water looks good applied in wheel ruts, but don't forget to apply a dash of gloss varnish to the the water surface. It will make them stand out and look much better. The deepest part of a rut will be much darker than the top edges of the rut where it has been drying out.
   Human intervention establishes what some areas will look like, depending on the nationality. For example you can give scenery a nice touch by looking on the www and locating old advertising posters. There are lots available. Reduce the size and print them out. When attached to the side of buildings they can add a feel of a particular location. French for France, Chinese for China etc etc. A rather bland model building can be made to look great with some period advertising signs. Propaganda posters reduced and printed out to attach, can give an excellent feeling of an era. When reducing, keep the posters small enough to look OK, but remember the visual effect is enhanced by keeping them large enough for them to be read and recognised for what they are.   Even a flat desert terrain can have notices at cross roads, so for the right look on a western desert terrain add a couple of army directional and HQ signs in the appropriate language. This is a small touch and one so easy to do, yet it adds an amount of flavour well worth your efforts.   A  civilian vehicle or two, a wreck, some debris, can also give a nice touch to your scenery. They don't have to be just objective markers! The use of these odds and ends can really lift the appearance of your scenery. An occasional civilian figure helps too.  

 Dirt. Don't waste your time trying to model dirt, if you can simply get some finely sifted real dirt and use that! I use real dirt on the base of my troops but also on my scenery. Its cheap and its real. I've seen some people go to amazing lengths to model dirt that is never as effective or inexpensive as the real stuff.  

There you go, back to me again...
Some great advice and tips there for all of you interested in building your own top level terrain.  It takes a lot of work at first to get the hang of it all, but its worth the effort in the end.
I really like the idea of the miniature posters. Whenever I see them on buildings, it gives the impression that they really are "miniature buildings" as opposed to lumps of plastic and plaster dumped on a board to block line of sight.

One of the wonderful things about creating terrain is that it gives freedom to create one's vision, in the same way that an artist uses a canvas. Unforetunately, the hobby is often marginalized as only belonging on game tables.
while this is certainly a great place for it, I've found that it works wonders in the classroom as well.
Over the last several years, i've been building thousands of Hirst Art molds for use in Elementary school classes, and have had outstanding success with them. Kids love to get their hands on them, and spend hours gluing them together and creating environments for them to exist in.
Bruce Hirst has an excellent series of ancient Egyptian molds, as well as Gothic stone, Fieldstone, and some Roman themed columns. This variety allows students to work in a variety of eras throughout history, in many different cultures.
For math curriculum, the use of shapes is covered, as the molded bricks are a variety of cubes, rectangular prisms, cones and so on. The art aspect is obvious, and it takes very little effort to work in a social sciences unit around the buildings as well.
When the buildings are done, the kids can put together living environments for their castles, again using basic terrain building techniques that add so much visual appeal to the settings.  Again, the kids can be challenged to represent resources found in the areas around their historical setting.
The only drawback is the time it takwes to make all the bricks. I'm fortunate to have a job that allows me to take the time when i can, which i am very grateful for. For anybody else it may be a challenge though.  Anyway, you can always buy my stuff... :)