What render engine do you use ? The technique may be very different from using Scanline or VRay.
As general tip, for grass i suggest to use a Full distribution map, applying random translation and scale. Since Forest Lite doesn't support Random Tint, you could add some diversity using different grass samples, modifying slightly the map's color or using several textures.
To get rid off the grass edge, use a thinner texture and smaller grass patches. If the texture included with Forest Lite doesn't fit your needs, you can create alternative maps rendering a 3D grass model with alpha channel, to get the texture and mask.
For VRay grass, please see this post that includes a realistic grass sample scene:http://www.itoosoft.com/forum/index.php?topic=654.msg2147#msg2147
About the trees, to get realistic results using billboards on closer views, you should try different textures and models to find the best fit for the view. But probably, for this scene would be better to use a 3D tree. In the Urban Park tutorial you can find a free 3D palm model.