Alright, so this is not a guide for how things are to be done in terragen, but rather a guide for composing your landscape scene using the tricks of proffesional photographers as a photography and landscaping in terragen has huge commonalities in that area.
First off, what you are going to need is some interesting landscape, this can be done in a number of ways, but always remember the details (so use the fake stones shader). There are numerous tutorials on creating landscape, using a third party app is probably the best so that you can mold it just the way you like it.
When positioning your camera, what you always should remember is to think about foreground, mid-field and background. A lot of people forget the foreground because they think that the mid-field and the background is such a spectacular view. You are not used to only seeing a mid-field and a background, which is why those images often look weird, only its hard to point out what it is.
In case you aren't sure what those are, here:
Foreground - The closest bottom part of the scene. The sky can be just as close at the top, but thats not it anyways. Try to have the camera close to the ground, or put some structures pretty close.
Mid-field - about 1/3 of the way up in the image up to the horizon. Heres often where you have the grand objects of interest, like some amazing canyon or mountains or whatever.
Background - This is your sky. Wherever it starts (you might decide to keep the horizon very high if you don't want to include most of the sky.
The last vital point is atmosphere. There are a lot of different factors here, light, clouds, haze and so on. What you are going to want is probably something unique, this you can attain by having a special atmosphere as some grand clouds (like luc's) or special lightning. What you always need to think of though is the sun's position which is very important. If you are out taking photographies of landscape with the idea of selling them to some magazine in your mind, the editor is not even going to look at them unless they are taken at either one of two times in the day. That would be morning and evening. This is when the light is not "hard" and direct, with mellow shadows and probably some cool rays. So the best scenes are the ones where you have the sun pretty low.