if you're going to use plaster, id make sure to paint and weather it first, for dirt, then spread a thing layer (diluted with water) of elmer's white glue. then sprinkle the static grass down on that. personally i also use celluclay, as its easier to work with has a longer drying time, and is less likely to shatter than plaster (if you use it though, make sure to add a few drops of dish soap to the mix, it helps keep it from shrinking and warping when it dries).
the technique JGAGS mentioned works well for taller grass, not so much for the small stuff. i also find though that gluing grass right to the celluclay after the grass in the holes sets, provides a more realistic look, and the dried grass acts as a stabilizer for the grass being glued on
hope it helps