I made a simple moss-covered wreath as part of my Christmas decorations this year.
I used a styrofoam base I got at a local craft store,
some fresh moss,
and a glue gun.
I removed as much dirt from the moss as possible and then just glued it onto the styrofoam base in small patches until the whole thing was covered.
Finally I pinned some ribbon and a felt deer to it and hung it on a door.
More pics can be found here: http://www.ourlittlebox.com/2009/12/diy-moss-wreath/.