Actually, I have found this combo works best:
Horticultural sand, washed clean of all its silt with distilled water. Mix it with peat (make SURE it's not Miracle Gro Peat!!), and perlite. Don't use vermiculite, it gets funky. Top off with a layer of live moss, if you can find it. If not PM me, I have some.
Somewhere on here I've posted some really detailed infor about how to mix this all, and make a terrarium - it had some really helpful info in it but I will have to look for it. When I find it I'll cut and paste for you!