The best to use is Pro-Mix. places like home-depot don't sell it but a good nursery or garden center should. It has no
"soil" (as in what most folks think of as "dirt"
but consists of peat, perlite and/or vermiculite, a wetting agent a dn a few other things (trace minerals, etc.) Some retain moisture better than others and I prefer Pro-Mix. Even if you get plain potting soil make sure it says "guaranteed sterile" on the bag. This will lessen the incidence of fungi, insects, weeds, etc.
Go to this site and scroll down to Premier Pro-Mix to read about it. Putting stones on top is great to help maintain moisture as well, the main thing is don't let it be soggy/constantly wet.
lele
Pro-Mix