I actually have many "techniques" so to speak, every time I do what I think will fit best. The two main ways I make the ground, however are:
1- Custom brushes - They can be either just plain simple brushes or texturized brushes with modified proportions, i.e. stretched horizontally so they give the impression to be in perspective.
2- Texture - I pick a texture, either photografic or hand made, and then adjust its proportions/perspective to match the scene, then I add details upon it. Texture are a great instrument to bring some realism into the work but they have to be used with caution, because they can mess up everything.
I can also use a combination of these two, making a base with technique 1 and then improving it with technique 2.