I'm using cocofibre bricks, its called Exoterra Plantation soil and is bascially finely ground coconut husks. I also have a small section of moss and some exoterra forest bark. These types of things are a little harder to maintain though. Many people will swear by newspaper, it works well and is cheap and extremely easy to maintain. Astroturf is also good. Just don't use sands or anything that is gritty and abrasive. Also no ceders since they can be toxic.