Chris Brantley has a page with advice on scratch-building wargames terrain. It's aimed at newbies in general, and DBA players in particular.
Arofan Gregory has posted TIERRA Y LIBERTAD, a set of free fast play wargames rules for the 1910 - 1920 Mexican Revolution.
There is no shortage of free wargames rules for the Colonial period. A good one is the Two Page Skirmish rules from Steve Winter.
Instructions for making a 15mm Aztec Pyramid from David Kuijt. I especially like his solution for making the murals along the sides of the buildings. These are impressive pieces of wargames terrain.
If you're going to play miniature wargames set in the American Revolution or the American Civil War, you've got to have rail fences in your terrain kit. Bob Bailey and David Glenn offer advice on making 15mm worm fencing in the Terrainmakers' Yahoo Group.
Here's how I make a lot of good-looking 15 mm split rail (worm/snake)
1. I use boxes of flat toothpicks, making sure to trim off about �" off of the pointed end of the toothpick. (Save these pieces.)
2. For the bases, I use the big tongue depressors. They're about 6 inches long with rounded ends. Craft/popcycle sticks aren't wide enough for the fences. The rounded end is also useful for combining sections at 90 degree and other odd angles leaving little gap between sections.
3. I also make up a few half-sized sections by cutting a few tongue depressors in half, and `rounding' the flat, cut edges. These come in handy for creating gaps and for surrounding those `odd' sized fields and pastures.
4. Take the fence bases, paint the top side and edges with green paint, then sprinkle or dunk them into the proper flocking material. Shake them off, and set aside to let the paint dry.
5. Once dry, you can start to make the fences. I put some glue on a flat surface (a piece of aluminum foil, piece of plastic or scrap cardboard). Using a pair of tweezers, I start laying rails by dipping each end of the rail into the glue and then placing on the base in a zig-zag pattern. Keep making successive layers until you have the proper fence height. (about 3/8" high)
6. Some types of split rail fences have small, vertical rails placed to each side where a `zig' or `zag' is created, crossing just above the top rail. Look at some old pictures to see the type of fence I am talking about. Use the �" long pieces saved from step 1.
7. For variety, I add some ground foam, small rocks along part of the fence base, and add some taller grass at the rail `elbow-shaped' areas, where weeds would have grown. This gives it a more realistic appearance. "Remember, it's the little details that count."
8. Either leave the fences a natural color, or give the wood a light washing with a brown or grayish color paint or stain to simulate aged wood.
I made about 50 feet of fencing in an evening, after all of the rails were trimmed.
Take flat toothpicks and cut them in half. Use the flatter half for the fences, laying them atop each other with the contacting ends glued.
I found that about 5 "rails" gave a good height. When dry, I sprayed them a moderate grey, washed them with a grey-black wash, then drybrushed them with a light grey, thus giving a quite good weathering effect.
I used the other halves of the toothpicks as the horizontal rails of post-and-rail fencing, laid between pairs of wood posts attached to bases, again with the contacting ends glued.
The flatter ends work better with the less supported nature of the worm fencing, while the thinner ends are very convincing as the narrower rails of the post-and-rail fencing.
I think I like the idea of the bases with the vegetation "extras" better than my "no-base" approach, although mine can be used at all times of year (I use a brown cloth for early spring and late autumn, a green cloth for warm weather, and a white flannel cloth for winter scenarios).