4 different objects for the grass, I used 4 to make it more random/natural.
1: Main covering grass object, covers most of the hill (high density).
2: Another main covering grass object with slightly different colour shade to give some variation. Use the populator in small size overlapping one another to give it some patchiness (high density).
3: Tall ones that pop out every now and then (look near the boundary of sky terrain you can see them), (sparse density).
4: Grass clumps that are taller/darker than the main covering objects to add random rough patches (sparse density).
That's basically it, pretty simple huh? no magic tricks.
edit: I also find it necessary to render with high AA (4+) settings when there are objects involved, it tends to give better results.
