You can try Texture Stamps.
Depending on what type of clay you are using, the stamps may be to soft to leave any detail imprinted in it so you may have to cover the area you want to detail in Gel Medium or something similar.
You can buy Texture stamps online and at some art stores, or you can make your own. One popular way is to take something like an avocado and use liquid latex. Layer it on and let it dry. Repeat this to the desired thickness (probably 7 layers at least) then powder it lightly as you peel it off. You can then press the stamp into the clay or the gel on top over and over leaving the desired detail.
That's the simplest thing that I can think of. I look forward to seeing what others do though.
Hope that helps.