Jonty has been a member of my online Model Train Club for 7 months now posted this idea in the members area:

I prefer grass carpet which is a simulated grass with a paper backing. It is hard-wearing and easy to apply to large areas. However too much of it can look a little too perfect (like a golfing green). What I do is weather it in places with a little bleach (watered down). Depending on how strong the bleach concentration is, the grass carpet with turn whitish, or a browny, or yellowy color. These colors can be effective on steeper slopes with little vegetation.

I have also used the Woodlands Scenic scatter grass and also make my own from a recipe that seems to change every time I make it. I often combine the above two techniques (bleached grass carpet with scatter grass) to get a very natural looking scene.