Do you import heightmap from other sources or do you use the built-in tool?
What sculpt-settings do you use on the built-in terrain-tools?
Hope to get some tips 
Mostly I create them with the built-in tool -> I use all of the sculpt tools with different settings. Sometimes the paint tool with a custom brush shape and a value of 0.3 and after that the smooth tool with a spherical falloff with a value of 0.07,… 
So all in all I use all tools with several brush shapes and strenght 