I agree with retrocity. playing with the presets are a must when you are trying to get the most out of the grass brushes.  If you download one make sure it comes with some kind of guide that shows you how you can use it.  Here is an image I made using the two grass brushes that come with photoshop CS.

When I use the grass brush I use a similar method that retrocity described.  After you get a feel of how to use the brush then you'll be able to have an easier time when trying to create different affects.