What I do, personaly, is determine how close my camera is going to be to a surface, (to determine grass PFX brush detail),
then I'll paint some grass in an area to verify that it looks ok.

Then what I do is save that brush as a preset.
Open a new scene.
Create a nurbs plane.
Cover it with my grass, and match/import my lighting from my target shot.

Create a 2 node or Orthographic-switched camera, and place it at the recorded height off the ground plane for a render.

Then I'll render the plane out at 2k or 3k depending on how much area of undisturbed grass I require for my shot.

Then I just use that as a simple 2d texture in my actual shot.

Looks convincing enough for my needs, and the render-time is quite fast.

Play around with the shader that contains your big 2d grass-map, like:
-add a SLIGHT bump for added detail during those moving motion-blurred shots.
-use a Blinn, or Phong instead of a lambert, or surface shader for that added "glint" from your bump for that "morning-dew" wet look.

Keep in mind, that if you want flowing, blowing, and bending blades of grass, fur and or PFX in 3d is the way to go.

But for medium-high-level flyovers, and fast renders, my technique has worked for me in a pinch

Good luck!