I used to use peat moss, which worked great for me. Two years ago I switched to ceramic soil after a discussion on the GGA List. Jon Boone recommended Seramis, which is a European product, and I tracked down Profile Aquatic Pond Soil manufactured on this side of the pond. It works like a charm. It holds moisture, humidity is perfect, and can be sterilized and reused. It's also known as Turfcare in some stores.
You can see it works - it's the media shown in this picture: