Here is a post that I made on the LDSIG group site, and I thought I'd share it here. The question was around how to make a 12" deep layout look like the scenery is deeper than it really is. Here is my reply:
Some tips are:
Make use of forced perspective. Use slightly smaller scenic items in the background to enhance the appearance of distance. You have to be careful with this one because usually to work it needs to be seen from a single point. Once you move to the left or right to look at it again the illusion is gone. I don't recommend this technique for long stretches of track, but it is excellent for a corner!
Completely hide the horizon line (backdrop to benchwork line) as seeing it allows the eye to compute real distance. A low fence or a tree line is very effective.
Raise the horizon line 2" as it causes the eye to calculate the horizon line as being further back. A slight curving upward fromt he benchwork looks nice.
Keep the track as close to the aisle as you can, but avoid parellel track to edge. Using flowing curves when possible. You don't want the viewer's eye to make calculations!
Use scenic elements to point the eye to where you want it to go. Carefully placed trees, buildings etc can block the viewer's eye from wandering around. Direct a roof line or a small foot bridge to point the eye back to the track.
Never put a structure parrellel to the backdrop. Always put it at some kind of angle.
Keep the track in front lower than the scenery in the back. Looking "down" into the scene makes it appear smaller. Looking up makes it appear deeper.
Keeping the background scenery plain and uninteresting forces the onlooker to focus on the train running through. Keep the scene sparse, and heavily detail the foreground.
Heavily detail the track! This is often overlooked. Have a scene that is just inches away from the backdrop? Throw a string of new crossties and spike barrells along the front of the track and see who notices what is on the backdrop. Very effective!
Use bland colors in the background and avoid bright red, yellows, etc.
Don't put readable signs in the background. Put them up close to grab the viewer's eye.
Use painted backdrop continuances, such as a road painted up to a higher horizon line, or a fence or river. This allows for the percieved continuation of the layout and gives the eye something to do.
That's some ideas...maybe the guys have more. Please post your comments!