When starting a project look to real life for details. If it is a piece based on something from the films put the DVD into your PC and take lots of screen grabs, the more the better. Get pictures of as many angles as possible. If it is more natural based such as trees, go outside and look at trees and take pictures. Notice the colour of it, such as the bark, you will notice that it probably isn't as brown as what you think. Look around the base of the tree and see how the ground looks. If you live near ruins or even a castle (such as I do) look the colour of the bricks. Take note of how the grass grows around it all. Get as many details as possible and refer back to all your pictures when building.
Planning is a key stage. Make a drawing of how you want your finished piece to look, a top down plan is a good idea. It doesn't have to be a work of art but it will help you in the long run as it will be easier to change something now at this stage rather than when it is in the building stage.
Think about the over use of your terrain piece, remember it is for war gaming. It may be the most detailed and accurate to the films piece but if you can not play on well then that may no be the best option, no one wants to play on a terrain board where the entire playing area counts as difficult terrain! So sometimes it may be wise to sacrifice some detail in order to have a better playing experience (of course if it is a display piece such as a diorama then knock yourself out with all the details).
When making Bree I originally planned to have the houses with interiors (in fact they are built this way) as my original intention for them was for a RPG/SBG hybrid I was working on but this of course was over kill for a SBG and would take far too long to complete so I have since decided that I will just concentrate on the outsides. Always remember what the goal is for your project and don't make it harder work for yourselves than it needs to be.
There are many different materials that you can use for projects. The one you ultimately use will depend on factors such as cost, quality and availability. Take making a cliff face for instance, you have many different options available. The most common you will see if polystyrene foam cut with a wire cutter then painted various shades of grey. This cheap and convenient as polystyrene foam is easily available as it normally is used in packaging to protect the contents. The down side is that it just doesn't look very good, but there are alternatives you can use to improve the quality. Woodland Sceneics do various rock moulds that you can cast using resin or plater-of-paris. The end result is much more satisfiying.
The downside is that if your cliff covers a large area it can get weighty. Another alternative is to use cork.
Another method is to expand on the first one. Before painting the foam cover it in tissue paper and PVA glue. This gives the surface a nice rock like texture and has all the advantages of being cheap and light. I use this method when covering large areas such as my Helm's Deep cliffs.
I am a big fan of Games Workshop paints, even when they dry out you can most times add a bit of water to them and get them back. I have some that I have had since 1997 that I still can use. The downside to them is that they are expensive. When I was more involved with the game I made a few trips to GW-HQ in Nottingham and on one trip there I had the pleasure of chatting to the Perry brothers and talk about terrain making and even they suggested what I am about to. That is for bigger projects where a large area will need to be painted use something like emulsion paints or better still poster paints that can be picked up rather cheaply from craft stores. This is what I use when painting terrain pieces. Be aware not all paints are made of the same quality. Some of the poster paints I have used in the past have basically washed off when they get wet again, this even applies when putting on a second coat of GW paint. Of course this can have a good effect, as it can break up the uniformity of a base coat, such as when I painted my Bree roofs, the black poster paint undercoat begun to mix in with the Codex Grey I used and then gave each tile an almost different grey.
Don't paint rocks grey. I know it is temping to do so as when you think of rocks you think of them as being grey but in reality they are not they are more of a brown colour, if you have followed the research tips you would of notice this. A good starting point for painting rocks is black undercoat, drybrush with Graveyard Earth and then a light drybrush of Kommando Khaki, you can always experiment with these to come up with various colours of rocks such as replacing the black with Scorched Brown (maybe mix in a little Codex Grey into the brown as a base).
On the same way don't be tempted to use all browns for wood, the most common I have seen is Scorched Brown basecoat then a drybrushing of Beastial Brown. Instead of the brown drybrushing try using Kommando Khaki, this will give the wood a weathered look, great for buildings (such as Rohan) and fences that have been exposed to the elements for some time.