I haven't came across too many threads on this topic so I thought I would share the things I have had the most success with.
I put my spare sphagnum in a large orchid basket that has a few holes drilled in the bottom and is filled with about 1-2 inches of perlite. The perlite is to keep the sphagnum just above the water line.
Next, I place the basket in a tray of water and fill at or below the perlite. This insures that the sphagnum is not submerged for extended periods but has plenty of water.
Under heavy flourescent lights, I find that I have had plenty of sphagnum to top dress many pots and also have some handy nurseries availble for various cuttings.
Here is a pic of my sphagnum garden/CP Nursery complete with red and green sphagnum. I just received the red and would really like to split it into two baskets if I only had the room. Under these conditions, sparse areas become dense in a matter of weeks.