Terragen Tips and Tricks
Render settings recommendations
Some tips on how to optimize your render times and quality thanks to Oshyan.
Ideas on how to make caves
Few ideas in this post of how to do caves. dandelO found using a plane object with a crater shader rotated, and Seth uses canyons converging
Waterfalls
From objects - A not quite finished idea on waterfalls, but has amazing results so far.
From clouds - a method of creating waterfalls from cloud objects with a painted shader.