I just use tap water, personally. Since they aren't a significant source of biological filtration, I'm not overly concerned with killing bacteria in them. Just be sure to let it dry completely before replacing. It helps to have more than one. I've heard of some people turning them inside out and using the laundry machine (with no soap) but I'd advise against letting a wife know about this!
Someone stop me! Please! But... not just yet...
55 Gal Tropical FW
55 Gal Mixed Reef