Hec,
At
Boon's, we prep our trees very carefully for his professionally produced and exhibited show. Here's a link to my article on the process:
http://www.bonsaikc.com/showprep.htm
One thing I left out in the process, is that we use a very fine black lava between bits of moss. It's put on after all the moss is there, just kind of sprinkled in any gaps, etc, and broomed off to make it look very natural.
I hope this helps...if you don't moss your trees completely, the fine lava makes a great top dressing where the moss isn't.