I use a kind of thick felt carpet.
Umm... Lemmmesee... Ah. It is called a "Lizard Litter Terrarium Carpet" made by ESU (Energy Savers Unlimited, Inc) in Carson California. esuweb.com
I keep a clean one handy (yep. Two per viv).
When one starts to be visibly or olfactorily (is that a real word?) dirty, every week or two, it depends... I take out the contents of the viv, wipe it down with a wet rag, and put the clean carpet in. Then I take the soiled carpet outside, rinse it with the garden hose, hang it up to dry.
About every other time I use dishsoap to disinfect it. When I feel the need I wipe down the viv with an ammonia based window cleaner (great disinfectant, but make sure to dry it ALL out before putting a herp back in the viv)
So far this works flawlessly.