I always add a bit of rough sand followed by fine sand on a base. Then after it dried up I paint over it with some drybrush methods.
In the end you add some bushes or shrub on top.
There are also basing ways to get a piece of slate and put it on top of the base.
With half of my models I also use some cork on top of a base to heighten it a bit.
In general I got all the stuff from here:
http://www.greenstuffworld.com/en/49-basing-materialsMaybe you like my bases or not, but here you can see what I've done:
viewtopic.php?f=50&t=31894(I suggest looking at the bottom of the page to see my most recent)