Terrain Painting - Add Depth from the Start

The default textures tend to be bland.  They're designed to work over large spaces without noticable repeating patterns, but this makes detailing a huge task.  How can we add depth and interest over the full world for a better initial canvas without using a more detailed but obviously repeating pattern?

Create a reuseable mottled overlay! Choose a second terrain paint to blend with your default.  Export this layer to a graphics program. Using cloud effects, randomly altered patterns, and fuzzy brushes, add the sort of natural variation you would expect of the ground in your world.  Use brushes with tiny speckles to create small-scale variations, and cloudy spots to make large sections lean toward one texture or the other.  Don't spend too much time on the first draft; test the scaling before investing effort in detail work.

Import your new image into your second texture layer and check it out from above and from a Sim's height.  Repeat the process until you have the look you want.  Save the final layer image for use when starting other worlds.  No more bland areas to fill in!  Now you can work on adding even more interest.