I use Sphagnum moss that I place into a container (Blue Bunny Frozen Yogurt containers work great! They are made of good plastic, and you pay the same price you would for a little tub over in housewares but with the Blue Bunny ones you get to eat the ice-cream first, lol!)
I moisten the moss, place it into the little tub (with a hole cut in the side and the lid snapped tight) and then I slide my snake into it so they know that it's available and I just leave it in there the whole time so they can decide to use it if they want.
I just check it every few days so that I know if it needs re-moistened or if they poo-ed in it and it needs cleaning (which has never happened yet, they usually go in a corner, not in their hides; at least mine do). I've heard that it can get moldy but it's never happened to me yet.
Oh, and with the moss (and any organic thing that you buy from the store that you want to use for your snakes) you should bake it or place it in the microwave to kill any bugs, bacteria, or parasites. I place the moss in a cup, moisten the moss with water, place it in the microwave for three minutes, rinse it out with cool water (it will be REALLY hot. I learned that lesson the hard way by burning my finger), and they squeeze it a little so it's not sopping and then place it into the container.
Here's a pic of the ice-cream/frozen yogurt containers that I'm talking about.