After five years of scratch-building terrain, I think I finally have an idea about how I can start building terrain that looks good on the table. Five years? Yeah, and my terrain still tends to look bad. But now I have a plan.
When I was first starting out I read somewhere that when you design terrain, you need to be consistent with the materials and paint that you use. For a beginning terrain maker, that seems an impossible goal. The twig trees with yellow foam "leaves" does not go at all with the lichen/toothpick trees. Nor does the house made out of thin card look right next to the one you just made out of foam. It's okay. When you're starting out you need to experiment until you find materials you like working with. Eventually you'll find techniques and materials you're comfortable with. Then the goal of unity seems simpler.
The to keys to unity are in order of importance: paint and materials. Settling on a paint scheme is the number one factor that makes your projects blend together on the table. For example, my current board is using a pine forest scheme, so my base colors are dark brown and forest green (spray paint cans for $1 US at Wal Mart). Materials are next, but not as important since some of the differences can be obscured by a good coat of paint.
To break it down it to manageable sections:
Basing - Paint and flock all your bases the same way. It goes a long way to making your terrain look like they belong together. Then it doesn't matter as much what the actual structure looks like (well not as much).
Natural terrain - Paint all your hills the same. Using a light gray for exposed rock? Then don't switch to a light tan for the next hill. Also when you make trees, used the same techniques for all of them. Don't switch from plain lichen to commercial flocked trees, they won't look right most of the time.
Buildings - With matching bases you can have a whole lot more variety with these, but it can help to stick with similar paint schemes. For more flexibility, its good to skip the base all together (if possible), but if you do, you won't have that as a unifying factor.
So how do you decide on a color scheme? If you have a friend who already has a table built for gaming then use his table as a guide for the color and texture for basing your projects. Another option is to look at nature. Most temperate zone terrain could be represented by medium browns and greens. Deserts are probably going to be browns, tans, and reds. Mountainous terrain might have grays.
Once you decide on a set of colors, you can go to a DIY store or other craft store (including Wal Mart) and pick up some cheap cans of spray paint. Then get some lighter-shade acrylics for dry-brushing. That's all there is too it. Then every time you run out of a color just make sure you replace it with the same color.
Eventually your terrain pieces will start to look like they belong together... but don't worry if it takes a while, I'm just starting to get it right.