I store mine standing back to front inside baseball card storage boxes and have never used anything in between. There's probably about 1/8 to 1/16th of an inch space between them. However, if I were storing them stacked upon each other, I'm like Shari above- I'd use something like a sheet of parchment to separate them.
I too store mine in baseball card storage boxes. I don't wrap and label until a few weeks before I need them or as orders come in. I always have labels ready to go so it's just a matter of wrapping them. It's served me well for almost 5 years now.