There are a few essential points to bear in mind when planning or building terrain. Think of your own terrain collection or the terrain you use at a club or friend's house. Which pieces do you use most often, and why? The ideal terrain piece has high game play potential, flexibility, good appearance, is tough, and is cheap to make.
If a terrain piece looks wonderful but is terrible to play on then you will find it stays on the shelf. Try to bear these points in mind when designing terrain:
What effect will it have on the game, and how does it fit into the rules? Do you really want a huge patch of swamp that is classified as 'very difficult terrain'? Maybe you do, but this will become a specialised piece that is only used now and then. Large open spaces such as a large landing pad will become a killing field because of the lack of cover, leading players to avoid it and leave the terrain on the shelf.
Make terrain that is interesting and stimulating and have game links in mind. Does it conjure up gamplay or scenario ideas? A simple pack of crates can be an exciting terrain piece if its capture will dramatically affect the outcome of the game, perhaps by containing a 'free' piece of equipment. Do you want rules for the contents of storage tanks, is the river water, sulphur or toxic waste?
Design the terrain with your game system and miniatures in mind. Check scale and get things the correct size - how wide does a road bridge need to be? How far can the average character climb in one turn, and do you want your ladder this height or higher? How big do the doors have to be? Do you want to be able to shoot over that wall, or provide a hiding place?
If forced to choose between appearence and gameplay (for example hills that look good or hills that a miniature will stand up on) go for gameplay at the expense of realism - ultimately the point is to play games, not look at terrain or drive model trains around it.
Terrain that can be used in a variety of settings will get most use. Terrain will be flexible if:
It is not tied to one race or game setting. The Ork fort on these pages is attractive but doesn't get used all that often because it is large and tied to one race.
It is not too big - large terrain pieces dominate the board and push out other terrain.
It is functional - a building that you can't place miniature inside becomes a hill by another name. This is why ruins are so good - you can allow an easy way in. If possible, make the roof of a building removable to allow access to the inside.
A gameboard filled with beautiful terrain adds enormously to the enjoyment of the game, but it will soon become battered and ugly if it is not up to the job. Think about how you will store your terrain before you make it. If you are going to use a certain size or type of storage box then make sure your terrain will fit into it. Terrain that is tough sometimes takes more effort than flimsy pieces - MDF or plywood bases are harder to make than cardboard, for example, but the extra effort will pay off. If you are making a relatively expensive piece of terrain or something requiring a lot of effort on your part then it is worth spending a little more time and cash and putting it on a tough base.